@steakster interesting possibility there with a 12au7 to 6922 adapter. I also thought about 6sn7 to 6922 adapters, but the tubes are place horizontally in the chassis, so there won’t be room for 6sn7s in there without being a bit creative. But the 12au7 with adapter may work! I already use 12au7s with my preamp, and have a few to play with...