Viva audio Solista mkII tubes & more...

I was wondering if Viva audio Solista mkII owners had any experience in testing different tubes in this amp instead of chineese ones. Especially if 6sn7 in preamp section does any difference?
Did you do any experimenting? Results to share?
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
I just bought a Viva Solista mk II and was wondering if anyone has any tube experiences to share? Thanks :-)