I did change the 6SN7, with one recommended by the supplier, Upscale Audio. I did'nt hear much difference. Definitely I did with the 845, changing to a Shuangung 845C, a bit quieter and better base. They really are'nt that expensive, bought direct from china at TNT audio.
I should say I have the mark 1 version. I am not at all clear about the differences between mark 1 and 2, apart from the cost