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


VPI Prime----Ortofon Black Quintet (low output moving coil cart)-----Plinius Koru Phono Pre( loading at 20 ohms/gain at 66 db)-----Parasound Halo Integrated -----Martin Logan Montis speakers which have powered bass cabinets.

Using any other source component I have absolutely no issues. When I play an LP and increase the volume past 10 O'clock which is a little loud....but not ridiculously loud-I at times get this huge wave of bass distortion. I do have bass controls on the Montis's...and I have turned them down into the negative, but I still get what I previously described occasionally. 

Any and all help would be appreciated. This doesn't always happen, but its frequent enough to drive me crazy.

Thanks in advance.

krelldog
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.