There were actually 2 versions of LS15, latter version has removable power cord, better caps, improved power supply. ARC used to upgrade the original but would be surprised if they still do. Though you could possibly haver someone do the cap upgrade. When I called around back then, was usually told it really wasn't a model to be helped much by upgrade tubes or outside mods...I used ARC tubes, but the ARC upgrade to the latter version was a very nice improvement...
ARC LS15 tube replacement suggestions
I recently purchased an ARC LS15 line stage to run with my ARC VT100 mkIII amp. Everything sounds great but I have no idea how old the tubes in the line stage have been in there, so I would like to change them out. Audio Research suggests the EH tubes they stock at the factory. I have friends telling to go NOS and I have reached out to a couple of dealers, and they wouldn't really make a suggestion. I know there are some folks on here that either have or have owned the LS15 that can give me some feedback. Thank you!