When turntables are used in conjunction with powerful subwoofers, significant steps need to be taken to stabilize and isolate the turntables. In the more recent decades, turntables have been largely replaced with digital media. This is because the bass demanded in modern music can't be recorded into or played back from vinyl records. The more bass there is in the music, and the more bass there is in the system, the more problematic turntables become.

Turntables operate on a fundamentally different principle from magnetic (ie tape) and digital media storage and playback mechanisms. Digital and magnetic media sources are totally isolated from the environment in which they are used. Unfortunately, that's very definitely not the case when it comes to turntables.

Back to the stone age: A turntable pickup needle is a stone, usually a tiny industrial diamond, glued to the end of a tiny rod. On the other end of that rod is a tiny magnet inside a tiny coil. Moving the "needle" moves the rod, and the rod moves the magnet, which then produces a tiny electrical signal from the tiny coil. This tiny signal is amplified by the turntable. It is then amplified again by the phono preamps. It is then amplified again by the mixer. It is then amplified again by the power amplifiers. (When turntables were commonly in use, amplifiers were much less powerful.) There are several points to note from this.

1) The signal coming from the turntable needle is amplified by many orders of magnitude.

2) The signal is produced by physical vibrations.

3) Sound is also physical vibrations, so the output of the subwoofers can be picked up by the needle and re-amplified.

4) When the subwoofers are close to the turntables, there is a greater chance of this happening, which is one reason why I recommended moving the subs to the corner of the room, away from the DJ stand.

Drop the needle on the record: That stone is dropped into the groove of a vinyl record, the sides of which have textures pressed into them such that when the vinyl is scraped over the surface of the diamond, it moves microscopic distances back and forth at exactly the frequencies (notes) and intensities (volume) that are intended to be reproduced. These tiny movements move the magnet at the other end of the rod and those movements are precisely amplified to reproduce the music recorded on the vinyl.

There are important points to consider here:

1) If the needle moves at all, by any cause, that movement will create a signal, and that signal will be picked up and amplified by each subsequent step in the signal chain.

2) If the turntable, the turntable stand, the stage floor or the hand touching the record moves, that needle moves, and a signal is produced.

3) These movements happen at very low frequencies, below the frequencies that humans can hear and below the frequencies at which speakers can safely operate.

4) These movements are inevitably much larger in scale than the tiny movements that turntable needles are intended to encounter, so the effect is that the signal produced by these unintentional movements is far, far bigger and more powerful than anything that could be recorded onto the record(s).

Within the speakers, limiters are implemented to prevent over-powering the woofers under normal circumstances. In addition, frequencies below their normal operating ranges are filtered out. With tops boxes, the filters are high enough to keep the woofers in them out of trouble from these issues. This is one of the reasons it's possible to determine that the source of the problems with your system are related to out-of-control signals at extremely low frequencies.

Even in the subwoofers, the very lowest frequencies are filtered out of the signal being fed to the woofers, but a balance has to be struck between providing plentiful low-frequency output and preventing every conceivable form of damage. If subwoofers didn't produce any deep bass, they wouldn't be subwoofers. (They definitely wouldn't be BASSBOSS subwoofers!)

Vented (ported) loudspeakers can produce decent levels of low-frequency energy down to what's referred to as the resonance frequency of their ports. Down to those frequencies, the resonance of the port does two things. One, it provides acoustical output, and two, it modulates (restricts) the movement of the cone. Below the resonance frequency of the system, the port is no longer restricting the movement of the cone. As the frequencies move well below resonance, the port begins working against the amplifier's control over the cone. When a powerful signal that is well below resonance is put into a ported system, it is possible for the woofer to move too far, damaging its suspension and/or its voice coil.

The filters that are used to reduce low-frequency output are best described as having a "slope" insofar as they reduce the level of frequencies below a chosen frequency more and more the farther the signal goes below that chosen frequency. These filter slopes are described in terms of "decibels per octave" with an octave representing the range over which a frequency halves or doubles. (40Hz to 80Hz is an octave. 60Hz to 30Hz is an octave. 500Hz to 1000Hz is an octave, etc.)

To put it in specific terms, the filter frequency in the DJ18 is 30Hz and the slope is 24 decibels per octave. If the subwoofer receives a signal at any frequency below 80Hz and above 30Hz, it passes the signal through to the amplifier unchanged. If it receives a signal at 15Hz, one octave below the filter frequency, it reduces the level by 24 decibels before passing it through to the amplifier. When music program is the source, this is sufficient to protect the woofer because there is no music content at 15Hz and the levels are consistent within the recording.  

However, if a signal comes in at 15Hz that is 24dB higher than the music level signal would be at 30Hz, the two signals would reach the woofer at the same level. If the woofer is driven by a signal at 15Hz anywhere near as hard as it would be at 30Hz and above, even occasionally, damage will result. Similarly, at 22Hz, the signal is lowered by 12dB, so a signal at 22Hz that was 12dB louder than the normal signal from a record or digital file would push the driver beyond its limits.  

We do our best to protect the speakers from every circumstance that may be anticipated to put them in danger, even well beyond "normal" but excessive movement and infrasonic feedback caused by unstable turntables is beyond the scope of the protection required by the vast majority of customers. This protection can be implemented but it's necessary for the customer to understand the dangers and limitations associated with implementing old technologies with new and powerful systems. When turntables were the primary source of music in clubs, bass was weak and DJs had booths to isolate the turntables from the speakers on the dance floor.

What can be done to protect high-powered, deep-bass subwoofers from turntables? The first, best, easiest and cheapest answer is to keep turntables out of the building.

If you do plan to use turntables regularly, it's best to go old-school and build an absolutely, solid immovable platform for the turntables. This is best done starting from a concrete floor and building the platform from concrete and/or cinderblocks and concrete. When the turntable stand is concrete and anchored to the floor with rebar or all-thread so that you can stomp-kick the turntable stand without skipping a playing record, you've got a good turntable stand. No part of the stage, railings or anything at all can be allowed to touch the turntable stand. In addition, some of the deepest bass would still need to be filtered out.

If you must use turntables, and you won't be building the above turntable stand, you have to give up on the deep bass. This isn't such a big compromise as it sounds because records can't store or reproduce deep bass. Essentially, if you're using records, you're not getting deep bass anyway, and you definitely can't access any of what they do have without the aforementioned turntable stand.

If you want to keep deep-bass capabilities available in your system when turntables are not in use, the deep bass must be filtered out when turntables are in use. This requires additional equipment between the DJ mixer and the speakers, into which the appropriate filters can be programmed. A "Turntables" setting would need to be loaded any time turntables were going to be in use. The MK3 line has these preset settings available in all subwoofer models.

To further reduce the possibility of problems, the subwoofers must be positioned and/or programmed so that a bass-null is created in the area of the turntable stand. Relocating the subwoofers is beneficial in achieving this.