Well that is very individuall and very discussed.Everyone is aming to find the most appropriate for himself sound.
When I bought my DSL-50-brand new from the music shop-bias was about 41,5mA(or mV in bias socket).If you set it's level too low you have a loss in the out power-but the sound is a little bit softer(accordnig to me).Some people like exactly that.
I have an amp with EL34 and power supply-450V which I have biased on 35mA-but it is not a Marshall.
Sometimes happens a misunderstandig e.g.-one JCM800 -50W which has a power supply about 380V - clearly needs a higher bias.
You can try to rebias it to about 40 mA and to bring it back to 33mA to hear the difference in the sound and to decide for yourself-what exactly do you want and how you like it.
Regards!
