Amplifier Enclosure Resonance

By three inches "wide", do you mean thick or high? If so, look for a VPI db-5 Magic Brick, whose dimensions are about 2" H x 4" W x 5" L and 8 lbs. It damps physical vibrations and absorbs EMI/RFI. Echo Audio in Portland Oregon has one for sale on their website, no price listed. Should be around $50.

FWIW, I'm a fan of covers off. It sounds better in my experience.

I will listen to the amp cover on vs. cover off. It should be an interesting experiment. Maybe I'm fretting over nothing. But long-term operation with the cover off won't work in my home. Cats and smaller children.

The amp has two rows of ventilation holes with a center strip, which is solid, that is 5" wide. I could likely accommodate something 4" wide. I thought it imprudent to have something cover some of the holes, especially if something were to melt and drip inside.

Hi imgoodwithtools:
 I did not notice any difference in sound between top off or top damped with the Walker discs but, in all honesty, I did not conduct a careful before and after analysis. My gut tells me it might be slightly better with the top off, although with the top damped by discs there were no longer any top vibrations that I could feel. My feeling is that if you have no pets or children to worry about and the significant other does not mind---off is the way to go for reasons of both vibration and ventilation. If all 3 conditions are not present, the top with discs is the way to go.

Does anyone make a product currently like this mystical VPI Magic Brick?