tube rush detection help

Sounds more like a noise which would be emitted by a small tube, not a power tube. Just to find out switch the small tubes from amp to amp and see if the hiss follows the change. I haven't typically (if ever) heard or been able to assign just hiss to power tubes. Snap, crackle & pop are more usual. FWIW.

When you say 'all the tubes', do you mean the power tubes as well. It is easy (relatively) to find a bad tube out of an octet, especially if you started out with a matched set so you don't have to fiddle with bias. You just switch half and if there is no difference you know the bummer is in the unchanged half. Then you change half of those. Again if there is no change you're down to two tubes, then just change one of the last ones and you should find the bummer. IF IT IS A BUMMER ISSUE. I'm suspecting it isn't, but at least you would rule that out.

Have you disengaged the sources from the amp to be sure that you are not hearing a noise floor or problem in a component ahead of the amp? Typically when someone complains of a tube 'rush' it sources to a tube in a pre-amp if the volume of the hiss is constant, and to a source if the volume changes with rotation of the VC (I've had chronic problems with a phono stage & low output MC cartridge) :-).