THe 6V6 option offers a lighter PS option. ~35 mA/valve of idle current. With 6-10 mA allowed for the front end, we'd be looking at consuming ~160 mA of 300VDC. No big deal for the St.70 power Iron.It is just an option in case the DH stuff was looking a bit complex( and it is a bit more compared to IDH valves like the 6V6 ). The 6V6 also has a fairly light heater load.
regards,
Douglas