Well the first thing to consider is the additional heater current draw, another 2 amps for 4 tubes. They draw considerably more screen current, tweaking screen resistor values will be helpful. And you'll have to add an adjustable bias supply. The stock fixed voltage is such that it would shut off the 34's. Or damn near. Otherwise peavey builds some pretty tough ****. You might not like how they sound, but they generally are built better then the other guys.. Or use to be.