Kinda glad that when ARC did the SE upgrade on my Ref 5 they kept the original aluminum top. IMO, I like the black metal top better than a see through plastic job.
As an aside, I've read comments from some who say that the tops on the Ref 5 SE and Ref 150 SE can cause microphonic issues. FWIW, I took the top off my Ref 150 SE. Can't hear any difference. I think it's audiophile BS.