Ah, I've had the same experience with JJ 6L6GC's. I don't think there's anything wrong with your bandmate's amp. The JJ's generally run much "softer" than other 6L6GC's. I bought a quad from a local tube shop, and tried them in several amps. In all but one of the amps I could NOT get the bias up high enough using the amp's trim pot. They now sit in my spare tube bag as, well, spares.
If I were you, I would see if I could trade-in the JJ's you bought for a quad with a harder bias rating. In fact, get the hardest set that they have. The JJ's are good tubes and it is worth it to try to find the right set. It that's not an available option, get a set of Svetlanas or NOS 7581A's, both of which should bias up just fine.
Oh, and by the way, I have my Model T's bias set at 80 mA per pair of tubes, instead of the factory setting of 60 mA. This is still under 70% of maximum allowable plate dissipation for 6L6GC's and improves the tone immensely - especially on the gain channel. Mine are Svetlanas. It is generally accepted in amp circles that Fender biases most of their amps "cold" to prevent increased tube failure rates. This is at the cost of tone.
Good luck with your friend's amp and tubes!
