Randyk,
I'll offer some additional theoretical musings. Like Stehno I haven't tried either of these racks, but I fully agree with his post. The Sistrum and Grand Prix are diametrically opposite in design philosophies.
I have a situation similar to yours, unsuspended TT (Teres) on a carpeted wood floor that's rather springy. The preferred solutions would be to:
a) move to a music room with solid floors or,
b) support the wood floor from beneath down to earth or,
c) mount the TT on a very solid wall shelf.
IOW, an unsuspended TT wants to have the most solid and rigid support possible. That's the whole point of the TT's design of course. To the degree that the TT can be moved, its inherently good transient and dynamic response will suffer.
So far this clearly argues in favor of the Sistrum. The problem of course is that d@ wood floor. If a rigid coupling to the floor would subject the TT to excessive feedback or make it vulnerable to footfalls then you may have problems with a fully rigid coupling. OTOH, putting damping materials of any kind too close to the feet of an unsuspended TT is a disaster. I've tried it and transients, dynamics and bass response are just destroyed. This, IMO, rules out the Grand Prix.
So what to do? My solution was to use a heavy, rigid rack but with carefully selected dampening beneath the feet of the rack ONLY. My intent was to isolate the rack from the floor while retaining as much mass and rigidity as possible. I'll probably even try some braces between the back side of my rack and the wall behind it. This may enhance lateral stability without significantly increasing floor coupling.
Purists from both camps may have at me for this approach. Each side is fairly dogmatic about not mixing damping with coupling. My preference is to analyze the requirements of a complex situation and use whatever mix of techniques make sense, regardless of dogma.
If I had to choose Sistrum or Grand Prix without auditioning, I'd go with the Sistrum as designed but I'd be prepared to try isolating it from the floor and coupling it to a wall if necessary.
FWIW, YMMV and all that. If Twl wants his HIFI Mod back I'll understand! Fire away folks.