A final question after reading more about these valves following your comments.
Is higher or lower gain better for hi fi? I tend to go higher gain in guitar pedals /amps to get more dynamism. Is there a preference for valves in hi fi amps? Is one purer or is there a payoff of purity v dynamics?
The Watford valve blurb on the GE ECC83/ 5751 says "The valve has a lower amplification of 70 mu as opposed to a 100 mu of a ECC83/12AX7. This makes it harder for the valve to distort and therefore an ideal choice when the best clean sound is required." Is lower distortion better?
I'm not sure you understand what has been said above about these different valves/tubes. It's not just about gain. It's about plate voltage, grid voltage, amplification factor, plate resistance, transconductance, plate current, load resistance, inter electrode capacitances to name a few. Each valve has different parameters (see their data sheets) and the designer will have taken those parameters into consideration when designing the circuit. You can't just swap these valves around as it will affect the performance of the circuit. Best to study the data sheets, or speak to an electronics expert to see what will be affected. It's no different to buying a high performance car optimised for 19" wheels and then trying to fit 21" wheels and tyres to it yourself and expecting it to perform as well. Of course it won't perform as well because the suspension and chassis have been optimised for 19" wheels. My advice is to keep the circuits and the choice of valves exactly as the manufacturer intended. Remember, just because the valve/tube can be taken out and replaced by the user doesn't mean it should be.
As a manufacturer 95% of the problems we see are user inflicted...