[yestertech]

Adi, I can only echo Frank's advice. As one, who built a 30W EL34 amp from scratch many years ago, I can testify to how much it hurt when I came in contact with the 400V HT one day !
Once you're in the hi power push pull league, high HT's and lots of heat is unaviodable
I will happily support you in your endeavours, and can supply bits, valves etc. but safety must be considered of the utmost importance. I built a wooden cabinet for my amp in the end as, due to the transformer used, the HT connections were above chassis.

As to choice of OP valve, you need to work backwards from the output transformer selected or available - this will dictate the choice of valves.

Don't underestimate the cost of a decent componenent - a flick through SOWTER's website will give you a flavour. Even secondhand, these things still go for fairly steep sums. For example, you may choose to use "TV" valves as Jefferey suggests as you have the 25 V heater secondary. The problem here is its quite difficult to get data for the correct anode load required for audio, as these were not published.( although it could be worked out ) This may lead to a figure for which no suitable transformer is avialable -
You see the problem Hope this makes sense ??

If you already have an OP transformer, the choice of valves will then be based on the transformer specs, and secondly on the heater volts available.
Is your mains transformer 25 volt secondary of reasonable capacity to support valve heaters ( it may be a 50 mA bias windning etc. )

Sorry to cloud the thought process, but this is no easy decision ( or project !! ) but it's still possible

Andy