If money allowed, I would definately go for the extra gain stage, and have it switchable in and out, to suit what you're playing at the time.
Other useful options could include cathode-biasing (automatic biasing) for power tubes. Or, if it's a fixed bias amp, you could have external biasing points on the back, so you could safely re-bias new power tubes with a multimeter.
Other useful stuff could include switchable voicing options, switching in and out appropriate resitors/capacitors, to give you different tonal voices for the amp. All pretty straighforward stuff for Martin.
An FX loop would be very, very useful too.
