Cartridge Loading and Compliance Laws

Depends on the washer... a nylon washer should probably be avoided- the coupling between the cartridge and arm should be as tight as possible, and nylon will squish.

Ideas why cartridge/tonearm mating may defy the predictions:
(1) The compliance of the actual cartridge sample could vary significantly from what the manufacturer has published, especially if the cartridge has aged.
(2) Many systems cannot reproduce much bass below 30-40Hz and so a resonant peak above 12 Hz is not much of a problem and/or the turntable is well isolated and in a very stable environment, so resonance below 8 Hz is also not a problem.
(3) Little tricks used in certain tonearms to dampen resonance and/or to reduce effective mass, such as moving the counter-wt very close to the pivot point.
(4) Inaccurate data on tonearm effective mass.
(5) When one spends a lot of money for a tonearm and cartridge, one is predisposed to like the result.

Great responses everyone, I really thought this thread might help to simplify things for those of us who are not gurus. But boy what a can of worms I have opened, it just keeps getting more and more complicated!...whew..lol Oh well, thats analog and audiophiles for you.

What about this:
http://shop.mapleshadestore.com/Tonearm-Resonance-Control-Kit/products/228/

I have a Lyra Delos cart (Approx. 12 x 10 cm/dyne at 100Hz; 7.3g) on a Pro-Ject 9cc arm (8.5g effective mass). For the most part it tracks well, but I occasionally get sibilance distortion in the right channel (even with too much anti-skate). Yes, my anti-skate is correct, along with alignment, VTA, VTF, etc. Would the brass weight help track the hottest sibilant grooves by adding mass to my arm? Any suggestions would be great. Thanks.

Wow, a 28gram brass headshell weight and 100gram counterweight add on?! Someone correct me if I am wrong but I think that is an extreme concept and probably not the best one. Instead maybe you could add a couple of stainless steel washers under your headshell screws atop the tonearm end where the cart mounts. If I had to guess I'd say you would be much better off going this direction over adding 128g of solid brass to your arm!

I could be wrong though so maybe someone with more knowledge can share their thoughts also.