The mid frequencies being out of phase from the high and low frequencies would have no real effect on your ability to overdrive the input tube stage of the amp, you need more gain and headroom for that. If you want to use the PCB for overdrive also the best thing to do would be to mod it to run off the +/- approx 16 volt supplies in the V5 for the stock op amp circuit or a 32 volt supply. Then you could take a gain of up to approx 10 at the summing amp IC1B without any op amp stage clipping. To me it looks like IC2A is not really necessary to the operation of the PCB you could just connect from IC1A to R7 and free up IC2A to use as a unity gain inverter for the mid frequencies or use it for a variable gain stage of 1-5. Also if you mod the board for plus and minus supplies then IC3 could be used for an inverting stage.