You might want to try repositioning the speakers with the cones under the preamp. I just personally learned that a tweak can alter the sound so much that you can't take your current speaker position for granted.
I have had some real lousy results with cones, misc rubber
feet, etc. under my Sonic Frontiers hybrid (one tube) preamp. Be VERY careful here, as my two years of experimenting did show it is far easier to make things worse than better, under a preamp particularly.
Out shopping, I saw these "pyramid-like" stones, smooth polished....four 3" sides at bottom, all coming to a single point at the top. Something about it just looked promising, so I bought four and tried them....the best I have found so far for my preamp. By the way, all four cost a total of 4 bucks...I found them at the dollar store!
Good luck....keep experimenting, in my opinion.