I need a quiet tube amp

Sounds like you have ground loop issue. Also, a 300B SET can't really drive the 87db proac. This sounds like equipment mismatch issue.

Check out www.decware.com. Their Zen triodes are DEAD quiet, and oh so engaging!

I am definitely going to check out the ground loop issue immediately.

But, as regards the speaker issue, the ProAc 2.5s have an impendance curve of about 6.5ohms to 11ohms and an extemely benign phase angle which, even at 87db, makes them a very easy load. I've tried a well regarded speaker line that is marketed to SET users (92db model) and it was more difficult to drive properly than the 2.5s. Also, my room is small (11 x 13.5 x 8ft ceiling) and I sit at a medium distance so this also makes things better. I also tried a stand mounted two way rated at 91db which did not come to life like the ProAc. Finally, my amp is extremely well designed with huge Tamura trannies that are desgined for better drive characteristics than most SETs.

I have been poking around trying to find something easier to drive and as musical as the 2.5s for the last three years but have so far been unsuccessful. But I am looking for a 2nd amp with more power and will use for "power music" listening.

I find my Audio Note Soreo SE very quiet and Manley Stingray dead quiet though not quite as nice.