That's awesome what you are experiencing with the GP feet, it's pretty amazing by just adding isolation one can get such positive SQ.
I have owned allot of ARC components over the years including the Ref3, great pre but the Ref6 offers substantial sonic differences across the sonic spectrum and will most defiantly put a huge smile on your face. Don't dare put one in your system unless you are prepared to buy :)
If it was my system as mentioned above my next upgrade would be before I spent $$$ on G5 cables I would get a Ref6 :) - paired up with the 75SE amazing combo.
With the Ref MM cables through out paired up with the Ref6 and Ref 75se the improvements I hear; noise floor drops substantially, everything sonic wise across the spectrum is just more realistic sounding, the sound stage is more focused with performers and instruments being more stable so this provided endless goose bumps listening - the base which to me is the foundation to music is substantial providing proper control without sounding mechanical - I'm very taken back and very impressed.
F.Y.I. Want to hear a nice improvement - place 3 Sistrum Audio points with disc under your Ref3 - http://www.audiopoints.com/audioPoints.php
Enjoy your journey!