Would an Isolation TT platform further improve my TT?

@rsf507 - You gets what you pays for

Vibraplane ELpF 95% at 10Hz - and a rather pronounced bump at its natural frequency of appx 2Hz - $2.5K plus compressor

Minus-K CT-1 99%+ at 10Hz (vertical), 95% or so (horizontal), natural frequency vertical 0.5Hz (good) but horizontal 2.25Hz (not so good) - $5K

Herzan TS-140 99% at 10Hz in all 6 axes, 90% at 3.5Hz, natural frequency <0.5Hz - $10K

In others words the extra money goes to dealing with 3-10 Hz and in more degrees of freedom. As 3-10Hz is where seismic activity is (pace Townshend et al) I’d at least go with the Minus-K as this is the sweet spot for performance vs price and on a rigid stand which isolates form horizontal transmission (i.e. translates seismic activity into mostly up-down) should perform well. Plus the Minus-K is completely passive.

Adding more mass to your setup is unlikely to address the one remaining source of vibration that you are not currently tackling -- subsonic or seismic vibrations that are being passed to the table from earth and result from traffic, construction or seismic activity.

Solutions such as a Minus-K platform or a Herzan like active isolation system would be worth exploring, but are both very costly. The poor man's alternative is suitably tuned springs, perhaps a set of such under one of the platforms you are looking at would be appropriate

Also adding seismic isolation will also potentially change the sound of the table

As @noromance noted 

I have always found that the more rigid the support, the tighter, more incisive and clearer the sound

The same is true for true seismic isolation -- you will get an even more clear, direct, un filtered and accurate presentation of the LP. Frankly what this means is that a lot of what many people like about analog ("warmth", "bloom", "space") is sharply diminished - if it's not present in the recording.

The reason is is because a lot of these apparently positive benefits are colorations from external vibration and taking it out at first listen can be shock -- LPs sound a lot more like digital in that regard after isolation. But the plus side is unparalleled L-R clarity, greater tonal variety, better micro dynamics and all around an ability to more closely engage with the disc. This really pays back on classical but if your taste runs to rock you may prefer the "before" result ... I've been able to do this test many times with my Herzan as you can switch it in and out and a fair minority of listeners prefer the sound without it on

@inna that is a nonsensical comparison. The task of a stylus retracing the groove is to all intents exactly the same as a microscope or scientific instrument and hence solutions that work for the latter will assist the former. This is one area at least where there is hard and obvious science and designers like Mark Dohman have integrated these approaches into their tables

you are right regarding subtraction however, for example subtraction of motor noise, as you can clearly see in the traces posted in my system ... 😉

and Geoff while I agree in principle I should warn that if you are thinking of using an active or spring based approach a rigid support is the way to go, lest the problem of conflicting spring Fr you referred to, or potentially (in the case of a Herzan) even feedback!

Just so happens Kuzma are showing a full range of passive and active platforms in Munich now including one model they distribute which is made by TableStable/Herzan
https://www.monoandstereo.com/2019/05/kuzma-at-munich-high-end-audio-show-2019.html#more