You probably could have been able to increase the output power by upping the HT and upgrading the output transformer, I think from the looks of it, you probably could have been able to easily get something like 100 Watts from out of two 807 Valves in Push-Pull, I've seen a single-ended Valve amp that uses a single 813 Valve and the HT was something like 1000V DC, or there abouts, but the amp was a high-end HiFi amp.