Can't really picture how you want to add the glass in but if I was you I would try some lead shot or fishing weights lets say inside balloons first. Place them aroung the bottom or on top and see if anyting improves. You can also use sand in balloons or baggies.

There is a little room.  I was thinking to glue it down with something gummy like caulk.  Granite or marble would be hard to cut to size and too think.  So, you like the idea?

ML

I wonder....It sounds like one of two things to me. Either your 400 lbs of mahogany, along with the improved anti vibration construction and materials of the 34i.3 are taking care of your vibration.Or there is something else in the signal chain or room that is not allowing the amp to be as revealing as it can be and making it difficult to hear the changes you have described. I can hear different types of feet with my system.

I know what you mean. I had a magical sound the other day and did some changes . . . and I have a different magic now. LOL. Isolation control is so complex.

One thing to keep in mind, is that the Rachel (at least the one I cracked open) looked like it had conductive paint on the bottom of the case - if that's the case, it acts as shielding and I would avoid scratching or peeling it while installing or removing any damping materials.

You might also try simply floating the amp on a big foam pad seeing how some good soft isolation helps. I've tried it on a few amps after it was recommended to me, and I was surprised by how well cheap foam isolated a heavy amp. Ugly as anything, but good sound.

The key is keeping any thumping or resonation away from the preamp tubes. Steve showed me this when he demonstrated to me how the Herbie's tube dampers really didn't do a lot for thumping and tapping. It's also why the steel plate the amp is built on is so thick.

If I were to do an amp myself that didn't have any shielding, I'd treat it like it was a guitar and use copper tape to line the bottom and sides of the box, and make sure this had a good mechanical connection to the metal top plate. Here is how it would be done on a guitar.



Notice the bit of tape going up to a screw hole for electrical connection, and the dot of solder electrically connecting two pieces together. (makes me think a copper sheet in the bottom might work better)

But also keep in mind, Steve didn't use lots of shielding in the amps, so is it really necessary?