Preamp Resonance Mystery

I removed the 4 corner screws from the top plate of my Wyetech Opal pre 2 weeks ago to check voltage at the capacitors. I thought I was imagining it, but now I am convinced its true: when I put the top plate back on but did not screw in the 4 screws that go in the corners, the sound improved noticeably. More natural, realistic vocals; clearer, more "present" instruments; and deeper bass. What gives? Do I do something to the resonances in the pre? Am I delusional?

(Adding more to the improvement a week later was substitution of top-of-the-line Herbie's Ultra Rx tube dampers in lieu of the old Hal-o model. God stuff!)

Curious as to any insights or similar experiences from the assembled masses.

Neal