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.
