Cheap valve amps are more often than not worth avoiding, a cheap valve amp outputting 50wpc I doubt exists and if it did I’d be asking a lot of questions.
Presumably you’ve asked this because you feel your speakers need 50wpc (although consider current output too), you like the valve sound and have a £500 budget?
A couple of suggestions, first have you tried any class A amps? They may get you closer to the sound you are looking for. Another alternative would be to get a valve pre-amp and pair it with a solid state power amp. Something like a Croft 25 Basic will set you back £400, if you don’t mind buying on the used market you could then pair with a SS power amp in the £100-200 region from the likes of Rotel.
I used a Croft 25 (one up from the basic) with a Quad 909 and am very pleased with the results, a lovely tone introduced by the pre with the SS power my 909 offers. The iron fist in a velvet glove.
I used to assume the Fatman amps were novelty items but I have been told they’re not too bad, although I believe the are hybrid designs. I’m not even sure what they cost but they seem to be marketed at a younger audience so I’m assuming not too much, might be worth investigating. No way they’ll be 50wpc though.