One thing I always wanted to try with my own Magnavox 9300 is implementing a Blumlein garter circuit for bias on the output tubes instead of the 2 tubes per cathode resistor setup it currently has. Not sure what it might do sonically, but it seems that it would bias the tubes more evenly and make matching them somewhat less important. Might be worth a go if you're already planning to play around with the bias circuitry.