Why do you have a poly cover?
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Yes, It is a poorly designed cover. There should have been a circular opening above the 6550 tube. ARC says that the poly cover sounds better than metal. I have a Ref 5se with both metal and poly covers. I can't hear any difference. ARC has had so many problems with the poly covers that you would think they would do something. The new Ref 6 also has a poly cover.
Complain to your dealer. ARC will send him a replacement metal cover. I would do it soon as I have heard that ARC may not get more metal covers when they run out.
I am a bit concerned about the heat build up inside the case when using either cover. I keep my Ref 5se in a rack and don't a cover. It has much better ventilation this way.
Theo, I can't hear any difference with the cover off. Though I have heard this theory for years. I tried this experiment years ago with my LS25 mk2 and could not hear any difference then either.
The problem with the Ref preamps is that the top of the 6550 tube is very close to the poly cover. This causes a slight upward circular bump in the cover. It still happens with proper ventilation. I have an open rack with about 5 inches above my Ref 5se.
That said, The Ref 5se is an incredible preamp. I can only image how much better the Ref 10 is.
I had the problem with my Ref2 SE phono stage. I thought I was going to have to buy a new rack as the one I have only has about 3" of space above the ARC and the next shelf. The poly cover was starting to bow above the 6550.
A cooling fan solved the problem. I think the fan is a good idea even if you remove the cover. Cooler operation equals longer life.
I have a Ref5 SE that has the same polycarbonate cover. It is sitting on top of my rack with the ultimate ventilation. The cover is STILL WARPED! It has nothing to do with adequate ventilation. Lostbears nailed it, it's the 6550 power tube. Flipped it over and in 5 minutes it bowed again. I am ordering a metal cover to replace the plastic POS!