An ST-70 with with the VTA driver board will do about 45 watts per channel in pentode ultralinear mode and 21 watts in triode mode at low distortion levels. Check the independent test link below for more information ..
VTA ST-70 kit independent test link
IMHO 600 volts is too high a voltage for modern KT77 or KT88 tubes to run. Running modern KT77 or KT88's at this high a voltage will result (IMHO) short tube life. The only output tube in a Dynaco type amp that can run at these voltages are the relatively new Tung-Sol KT120 output tubes. If you are considering as you say > "building another ST-70 with this VTA driver and running higher operating voltage" then you should consider the VTA ST-120 amp kit. The VTA ST-120, although conservatively rated at 60 WPC, is really a 70+ WPC amp.