Pops and hiss from Sonic Frontiers gear

Albert is obviously experienced at this...
Whenever you hear a "pop" sound, immediately suspect a DC level-shift or "offset" as he so puts it. DC is a very bad thing to have in your signal path at anytime, it can quickly destroy things in a big way!

Thank you, Albert, this was exactly right. I used a kitchen spatula to tap the tubes, holding it by the rubber blade and tapping with the wooden handle, volume way down. The first 6922 in line on the right channel (and none of the others) produced a musical "clink!" when tapped. Replaced it with a spare, hooked everything back together, and no more pops or hiss. Excellent advice--many thanks!

The problem is most likely a tube in your preamp failing with voltage "offset."

You may test by tapping each of the small signal tubes in the right channel with a wooden or plastic ( NON CONDUCTIVE ) dowel rod. The culprit should show itself without your having to replace every tube.

The bad tube will most likely reveal itself in a big way, so have the volume turned almost completely off. A safer method would be to listen through headphones to avoid the possibility of sending catastrophic signals to your amp and speakers.

I have had problems with NOS Siemens tubes myself. I love the way they sound, but all too often turn microphonic, or fail in the way you are describing.

Let us know what comes from this so we may benefit from your test. PLEASE USE CAUTION if you remove the cover and tap tubes as I suggested. Lethal voltages are always present in any preamp, especially tube units where voltage is EXTREMELY high. You should know if you are confident of your skills in this matter. If in doubt, swap all the tubes from left over to right with all power off and see if the problem moves to the other channel.