Arc Ref 5se owners

I owned a REF5SE that was a dealer demo. It also had the lexan cover which was also warped. That 6550 does generate a lot of heat. It also burned the board, which Audio Research replaced as a warranty repair. The manual does not indicate that tube should be replaced at 2500 hours, it only states replace tubes at 4000 hours which refers to the 6H30. The REF6 manual does mention the 6550 replacement at 2500 hours which helped me get the warranty replacement of the board. So not knowing when the tubes were actually replaced, I strongly recommend replacing at least the 6550 as soon as possible. Also I did order a metal cover and did away with the lexan cover. They say it sounds better with the lexan than the metal, but if that’s your motive, just don’t use a cover if environmentally possible. 
And not to start a range war, I would only buy tubes from Audio Research. I asked them about a dealer that claims to be AR approved and was told they were not aware of that claim. You may call AR and ask them, as you may get a different answer. And hopefully you can still get a metal cover, and place the unit where it has plenty of room to connect the heat away from that tube. I also have an infrared thermometer and was getting well into the 200 degree range. 
One last note, it is a great sounding pre amp, enjoy!

Yes the power tube runs hot, my cover is also warped.  I took the cover off and put the screws back in the holes.  When I turn it on I remove the cover amd replace it when done and the amp has cooled (just laying it on top of the screw heads.  Also put a micro fiber cloth over the top to keep dust out.  

Regards,

barts

Audio Research definitely misjudged how its products are used when it put the plastic cover on its Ref 5 preamps. My cover has never deformed, but I keep my preamp on the top of my rack, so it is well ventilated. But many users allow only a few inches of ventilation - that can make the cover prone to warp as well as introduce other problems.

I also use an ARC Ref Phono 2SE as a phono preamp that is in a rack just below my turntable. It has only about 4 inches of ventilation above the unit, so I use an AC Infinity fan on it. That has prevented any damage to the plastic top and keeps the unit cool. The fan can't be heard from my listening position.

I had a Reference 6 twice. The first one (maybe an early model? it would have been made ~2016), came with a flimsy-ish acrylic cover that quickly deformed (sagged) from the 6550’s heat even freestanding in open air (nothing above nor around) in a cool room. That was a bad cover design. It looked awful, but did not cause any issues of concern other than that. ARC shipped a metal cover for free, but it was extremely thin gauge steel and rang like a bell. 

The 2nd Ref 6 had a much thicker cover with beveled edges that did not demonstrate this problem. 

Now this is off topic but I also wondered what their logic was for putting this same cover on the phono stage. Their Ref 3SE phono made demonic noises if you had a plasma TV on (even in a next room) - of note this only affected High Gain mode. Surely a thick metal cover (with damping applied because...tube phono stage) would have helped shield those JFETs? IIRC Fremer mentioned this in his review, so it wasn't just a one-off issue. 

OP,

Audio Research has experimented with the covers for their preamps for a long time. Originally they were metal... they found out that the plastic (acrylic?) sounded better... and that is why they made the change. They probably experimented with different thicknesses as well. This is common after a new model is nearly complet, or even... to experiment with the fit and form to optimize it.