Ah the 6x5, thats a tube often used by Audionote/Quortrup.
Punchy little buggers! Hehe, I'd say that Bendix 5852 would be the best...by far.
They are about 50$ a pop... Lasts a really long time!
You are trying to get a really huge dynamic wallop out of your powersupply I gather?
Yup, thats my suggestion, the Bendix.
Sits in the Voyager Probe (the spacecraft, was developed for this kind of abuse...) And in the Apollo moon program rockets/vehicles (I'm told.)
Does not get any better than this one!
The effect of this tube as a rectifier will give the sound a very spacious and airy quality.
No noise, and DA-BOMB in dynamics...

Clear as a bell!
Imperial