Thanks for all the input and suggestions chaps. You've come up with some very interesting points.

From what's been said I'll definitely stick with AC heaters on the output valves as the standard supply is centre tapped and I can't hear any hum what so ever through the speakers.

On the subject of the input valve DC heaters; I'll definitely upgrade the diodes on this. I've seen a good guide that shows how to do this including a pi filter to keep the correct 6.3V output.

Would there be any benefit to using a regulated supply here instead of the above?

Cheers.