A friend of mine in Serbia has a novel solution for getting the high amperage filament voltage required for the heavy duty valves. They start life as quality electronic transformers for halogen lighting. Depending on the brand, they have HF AC at over 30Khz. The second harmonic hum (>60KHz) he reports as not audible and doesn't get through the output transformer. The advantages are very high efficiency and no audible hum - even with 10V valves. Here is a link to the project. You need to scroll down about ¼ way down the page. It only involves rewinding the secondary on the toroid transformer and adding the cap
I have built one of Alexs amplifiers the RH307A Super and it is a solid design, sounds great and works essentially immediately. As he has a whole family of these amplifiers it is easy to mix and match for multiamping. The technical documentation makes DIY really easy!