First order, put in a pot per tube that lets you set the DC bias
current in each tube independently.

Second order, add to that a pot that lets you unbalance the phase
inverter outputs to equalize the AC gains of the output tubes as well
as their DC operating point. 

Yes - for safe operation you don't want your tubes biased so that
they either have too little idling current and sound harsh, or too
much and overheat.

You'd also like them to be DC-current-matched to a reasonable
degree, as this ensures that your output transformer isn't driven to
saturation on one side before the other. 

Beyond that, you'd like to match the AC gains.

There's a large section on biasing in the Tube Amp FAQ at GEO -
as well as info on many other aspects of tube am