Tonearm isolated from platter in separated tower

I am back to arm-board.

The TNT is suspended from outside generated mechanical vibrations by its suspension. The "new" tonearm platform is not.
Generally spoken it is not a good idea to isolate the tonearm from the suspended chassis.
The TT now is suspended - the tonearm is not.
Where the cartridge's stylus meets the groove the two "systems" meet.
The mechanical dilemma which does create - besides others... - the shy bass line, is obvious.
If my short explanation is not enough to convince you, I suggest you talk to Harry Weisfeld - he will happily explain to you why this is not a good idea at all, to have tonearm and platter/basis working on two separate resonance frequencies.
In physical means there is always a relative movement between the tonearms base and the platter - .... to quote Captain Jack Sparrow....... not good, not good....
So - there are clear and evident mechanical reasons for the sound you get right now.
Get back on (arm-)board will be the best (sonically and in theoretical physical terms) solution.

The tonearm is already mounted on a heavy support. I will try to increase weight. Thanks for your response.

Such isolation decouples the two elements and is likely to affect bass response. That (and cost/complexity) is why it is rarely done. You might try mass-loading the tonearm mount to match the resonant properties of the platter support.

Kal