Bass distortion-Playing LP's only,,,,please help

I would advise against the use of filters.  Yes cartridge loading can be a real issue and 20 ohm loading should be increased to 100 ohms and see (or rather hear) any difference.
Secondly, most good preamps will pass DC which will of course create havoc with your woofers.  Some woofer motion with normal vinyl playback will be present on a fair amount of material. 
So as presented earlier, TT isolation is paramount + proper cartridge loading.
Best of luck,

I get the cartridge loading, I have a lot of choices with my Plinius Koru.

Without sounding stupid...and I've never proclaimed myself an audio expert....

What exactly is Turntable isolation???  The first thing that comes to mind is a 2-3 inch platform underneath the turntable. I'm all in on this. I've certainly made the investment in LP playback. What else am I missing in terms of isolating my turntable???

As always I truly appreciate the help and guidance of other audiophiles that know a helluva lot more about this stuff than I do.

The Koru is a great phono stage. I've heard lots of good things about its sound quality. If the nearest preset load setting was 120 ohms I would cheerfully go for that but it's likely that 100 will be available.

Interestingly, after I posted I read a review by MF of the quintet and he tells that although they started at 50 they eventually opted for 100 ohms so it looks like, on this occasion, my x20 guess worked better than the usual choices ;)
Can't guarantee that'll work every time but since most go for even higher values I'm very much in a minority anyway! :)
(I'm currently using 220 ohms which for me gives a nice balance on the Delos)

Secondly, most good preamps will pass DC which will of course create havoc with your woofers.

This statement is entirely false. No preamp made passes DC!

Regarding loading:

If loading is affecting the sound, it suggests that the phono section is unstable with RFI at its input. Once corrected you will find that the loading no longer is needed.

The loading is used to detune a high frequency resonance caused be the inductance of the cartridge and the capacitance of the tone arm cable (inductance and capacitance are the building blocks of Radio Frequency circuits). This resonance is usually well above the audio band and can occur at several MHz.

Many circuits don't sound right in the presence of RF energy so eliminating it or the sensitivity to it (which yields greater benefit) is quite audible.

I discussed this issue with Jonathan Carr of Lyra fame a couple of years ago at Munich. We both had come to the realization that low impedance loading of a cartridge is probably adversely affecting its compliance, and so it likely reducing its performance in some tone arms. The idea is damping, not unlike how an amplifier can damp a loudspeaker.

So if you chose a cartridge to work in an arm based on its compliance (and resulting mechanical resonance), loading the cartridge may throw things off a bit.

Dear Ralph,

"...is probably adversely affecting its compliance..."

I remember hearing about that theory at the time and thinking it sounded "ropey". I have great respect for JC but how resistive loading could influence compliance (physical/mechanical) in a significant/audible way left me feeling slightly sceptical.

Has any more been published about this?
Cheers,

Bill.