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.
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.