Nik can add on a external biasing modification. Makes it much easier to rebias when trying different power tubes. Just need to get yourself a decent multimeter and you can do it youself. It's really easy:-)
I have a Overtone on order with the external biasing pot...when it does arrive I have a matched set of NOS GE 6l6's that I'm going to try out. For the pre amp section I have a set of NOS 12ax7 Mullards.
Sometimes current productions tubes sound better than the old stuff...and they are much cheaper!!! Part of the fun is finding the right tubes for your amp
Good luck!!!