Tube pre-amp is fine to remove lid. Solid State amp is a very bad idea. The SS amp bias is set according to temperature. Remove the lid and the amp will be all over the map in regards to stability/performance due to too much heat loss.
This gave me a bit of a start since I had not heard of such prior and because after I had Steve McCormack do his Platimnum mods to my DNA-225, I had told him of my plans to install a 3/8" thick piece of smoked polycarbonate as a replacement top plate. I drilled mnay holes (in the pattern of the stylized McCormack "M" logo) and additionally have the piece stand off above the sides all around via 1/4" nylon lugs. Steve thought it sounded a good idea. After reading this, I contacted SMc Audio today and got this intial response from Steve's great right-hand man, Kris Jeter:
"The DNA amplifiers bias remains very consistent and within the optimum performance window with the top cover off or on (unless it's subjected to gusts of cool air in an otherwise stable environment) and since removing the metal top cover can result in better sound, I would say that your setup with the polycarbonate top cover is optimum."
I subsequently got this response from Steve:
"A "well-designed" bias system should track thermally, and should be able to keep up with any gradual temperature shift. It is true that removing the topcover would allow air gusts to make the amp's internal *air* temperature less stable, but bias systems typically track the heatsink temperature, and these are usually external, anyway. It is true that transient variation in air temperature might cause relatively rapid changes in the operating point of some smaller devices not linked to the bias system, but this type of sensitivity will vary a lot from one design to another. I would say that thermal stability is a good thing in general, but most of us don't have strong winds blowing through our listening rooms, so I don't see removing the topcover as a problem. In fact, I do it all the time, and I don't hear any problem with my designs."