The upgrade from the MKI to MKII is a major step up in perfromance. I own the MKII and prefer the tonal balance of the MKII to the MKIII which uses an hybrid input section (FET/6H30). Recommended output power tubes would be the Sevetlana 6550C. You should purchase a matched Octet. The input tubes (6922) must be very closely matched. I recommend purchasing the input tubes (6H23) from Upscale Audio. However, if you change the input tubes you need to propely adjust the bias which is not a trivial undertaking. I have done it my self and it is quite time consuming.