Best isolation platform for a turntable with suspension?

Your Avid TT, utilizing a floating sub-chassis, would not do well with the Symposium Segue ISO for the very reason that internal springs combined with external springs is not a good combination as they would "fight" one another.  My suggestion is to get in touch with Peter at Symposium and discuss their other options.  The standard Segue platform (no springs), when "coupled" with your TT is what makes for the performance enhancement.  The platform uses a constrained layer design and is an excellent isolator itself, preventing external vibration from making it's way to the TT.  But the other half of the equation is to drain internal vibration out from the TT.  The basic premise of the floating sub-chassis is to isolate the playing mechanisms of tonearm/platter/cartridge from the vibration of the motor and main chassis, as well as that from external vibration.  However, it isn't a perfect solution and there still remains internal vibration that, ideally, you want to drain from ever reaching the playing mechanisms.  A direct coupling with the TT, known as a mechanical ground, is the solution for this.  With Symposium products there is a less expensive coupler to use, such as the Fat Padz.  But the far superior device is the Rollerblock Jr or Jr +.  The combination of an isolation platform with a mechanical ground delivers outstanding results.  Depending on the size of the platform (there are several sizes available to accommodate different sizes of TT) and your choice of mechanical ground device, the combined cost can range from $250 to $550.  Everyone has their own opinion on cost/value return on investment.  For me, spending $600 on a Segue ISO and Rollerblock Jr + was an excellent value in what I got for what I spent.  The results went beyond my best expectations.  Money well spent.  
Best to you on achieving your performance solution.  Be healthy.

OP, Symposium also make cup and rollers.

You might also look at Lessloss Bindbreakers, no direct experience myself but I have read good things including use with turntables 

A spring suspension engineered into a turntable is the best isolation. You already have that. You probably just need to get a more rigid rack underneath it. Cheap flimsy racks (e.g. VTI) are the absolute worst enemy to quality turntables. At least 4 posts, NO to the stackable / modular shelf designs, look for a wider footprint, don’t go too tall. And filling the legs with sand / shot won’t fix an inadequate rack. Honestly you cold probably do a lot worse than your current support, but it also wasn't designed for high-end turntables. The Symposium racks might be worth a look. 

Throwing isolation products on top of an improper rack support isn’t the way I’d go. As mentioned before, you don’t want to stack spring systems. And if your flooring is extremely poor then you’d have to fix that before anything else but sounds like you’re OK there.

I just had another one of those moments where its hard to stop smiling.  I have my TT on a wall shelf and have had a chronic issue of getting feedback/rumble at higher volumes.  I got used to it and hadn't tried anything until today when I opened my Delos box.  I picked up the 22 in wide by 1.75in thick used from The Music Room.  Wow!!  The rumble is gone!!  I am only 3 songs in but am so excited...I can't wait to experiment more and do some critical listening later this evening.  What gave me confidence in this working was that I would only apply slight upward pressure on my plinth and the problem went away.  There are many options out there but this beautiful walnut block looks pretty sweet and is very reasonably priced for the market.  Happy listening and kudos to isoacoustics!!