Send it to PRS. If it’s under warranty, they won’t charge you. Sweetwater are very uninformed about PRS’ warranty, obviously.
For goodness sakes, this isn’t rocket science!
Also, vibrations coming from the cabinet can cause tube rattle, so it’s worth trying taking the head off the cabinet. Tube rattle is generally from power tubes. Since gain channels are bassier, it makes sense that there’d be more vibration transmitted to the head from the cabinet.
Tube rattle is sometimes there because they aren’t making new tubes as well as they made the old tubes. Lots of internal parts in tubes are made very poorly and cheaply now. That’s just the way it is, unless you buy NOS tubes.