I've not listened to the Ref 1. But, FWIW, the two tubes I loved in my SP10II but which, unfortunately, were not rugged enuf to survive, were Amperex Bugleboys and EI6922's. The former are a tad pricy now and the latter kind of hard to find. I'd probably go with the Amperex.
Something else to check out also. In my pre-amp only the front two tubes are critical noise wise. The last two positions are not noise sensitive so you can get a lower grade of tube and save some money. I've never played mix and match but that is a possibility.
Oh, FWIW, I've been using 6h23eb in the line stage of my SP10 - they are, esentially NOS Sovtek's - maybe a tad warmer but not much, but much better tonally than the new production Sovteks.
Something else to check out also. In my pre-amp only the front two tubes are critical noise wise. The last two positions are not noise sensitive so you can get a lower grade of tube and save some money. I've never played mix and match but that is a possibility.
Oh, FWIW, I've been using 6h23eb in the line stage of my SP10 - they are, esentially NOS Sovtek's - maybe a tad warmer but not much, but much better tonally than the new production Sovteks.