I use the stuff from Parts Experss or MCM with excelent results. I also like 1/8" thick Armstrong floor tile secured with a heavy dose of contact cement. Best results if one type on the inside and the other on the outside IMHO.
Bituminous Damping
With all the tweaks I've seen discussed in this forum, I've seen nothing yet about bituminous damping material. This is stuff that's sold for sound-proofing car interiors, and is sold at auto supply stores in the form of rectangular thin sheets of a square foot or more. What I've used came from Germany and is 2 mm thick, but there are similar US products. I've used it mostly on CD players, transports and DAC's, sticking it everywhere I can find room for it on the inside and on the bottom outside. It is self-adhesive, but with my German product, I had to use my own floor/tile adhesive for the underside of the lid--when I didn't, it sometimes fell off. It made quite an improvement in cheap CD players I treated for friends, though that was easier to hear in my system than in theirs. I don't know how much it helps DAC's or other components without moving parts, but for CD players, transports and DAT's, let me recommend it as a highly cost-effective upgrade.