Go to [http:cartridgedb.com/]. They have all the specs for proper setup.
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You've already done the first thing, which is add weight at the headshell. Second would be to either add damping fluid, or lower the paddle deeper into the fluid, or both. That doesn't do the same thing as raising effective mass, but it simulates it. Third would be a tonearm wrap; do a search and you'll find plenty of info on both commercial products and DIY solutions like teflon plumbers tape.
With that said, I did all those things to my Well Tempered Record Player arm to try to make it sound right with my Denon 103R, and it was all for naught; it just wasn't a good match. It was much better in a heavier, fixed bearing arm.
Your Shelter has a slightly higher compliance than the Denon, so your results may be different, although there's always a dispute about the actual compliance of the Denon.
I second http://www.cartridgedb.com use the interactive tonearm vs. compliance chart.
100 ohm loading may be a bit low for a 901. (Bloaty/mushy bass? Highs don't sparkle?) It has an impedance of 20 ohms. Using the 25x rule-of-thumb = a ballpark optimum load of 500 ohms (although I usually find that a bit high with most phono stages, and certainly one as good as a CAT! I'd start at 300 ohm and work my way up until I started losing bass response and then back off 50 ohms or so. (Cartridge db user reports suggest 470 ohms BTW)
The compliance/effective mass may be a problem as David suggests but more damping won't help. That TA has a rather low effective mass for such a low-compliance cartridge which also doesn't weigh much itself. It's not that far off though. You need to increase the cartridge weight by a couple of grams, and you'll be right in there. I'd recommend using (heavy, yet non-magnetic)) brass mounting hardware (that'll add at least 1.5 gm). That, along with proper loading should be enough, I think.
In the old days you could get non-magnetic weights for the tone arm so you could take care of this problem. You could mount them between the cartridge and the headshell or on top of the headshell.
I've not seen anything like that in years, but it is a simple solution for an otherwise vexing problem.
Dear Rada: Other what you already do it ( and you can go on to some limit. ) there are somethings that can/could affect the bass performance ( other than VTA that you already try it ): azimuth, VTF, on target cantilever align ( not the cartridge body ), precise tonearm set-up and overhang.
Of course that a different phono stage can/could makes a difference over your CAT but like you say this is: out of question.
You can try on the load impedance but if you go from 100 Ohms to 300-500 Ohms and you hear a difference in the bass performance other than a little change on SPL then you have to change that CAT because that kind of change on impedance IMHO can't/could't affect the quality bass response.
Regards and enjoy the music.