The 5R4 is OK to use It can handle 250ma it does have a 69 volt drop at this current. The amp draws 220ma if using 6AS7s
The high voltage drop will change the sound for better or worse I don't know. It is according to the phones you are using.
Just don't use those boutique 5U4s Sophia or EML they will blow up I think they are mostly for looks.