Audio Research LS2B Mk 2 has a hissing noise

It Could be a tube going south? Flip flop the tubes ( right and left channel ) and see if the noise travels to the other side..If so you need a new tube set..Probably bad tube...Good luck

Thorman - The LS2 only has one 6922 tube.

It's easy to drop in another tube and see if the problem goes away. If not, expect $100-200 for service plus the round-trip shipping costs.

And if all is ok after a few minutes, why not just leave it on all the time? Sovtek tubes are cheap and will last you a few years before needing to replace this.

Like the LS1, the LS2B is a hybrid, meaning that there are mosfet followers after the triode tube which further amplifies the signal. It could be that the mosfet follower in the right channel has become noisey. The possible cause could be that the preamp was not unplugged & allowed to sit for a few minutes before tube rolling (a precaution not to be taken lightly), there is a good chance that this transistor could of been compromised. But definately try another 6DJ8 tube before coming to any final conclusions. By the way, ARC is not cheap for repairs!