About to `bay my early seventies Mk4 50 plus, also a B, & I am hoping it will get £400-500 as most of the recent sales have.
£450 is frankly a realistic price.
The valve thing is a bit of a red herring, as you have to weigh up the age of Mullards in an amp like this vs. newer, albeit possibly inferior, modern valves.
A clapped out Mullard will sound worse than a new noname.
I built a clone of a 57 Fender 5E3 deluxe amp recently & up against an original from 57, mine sounded 99% as good, even with new JJs in it.
Unfortunately it also has all the inherent design faults of the original!