I owned an SP-10 for 8 years in the 80s and found the RAM tubes to work very well. In the later years these were 6922. When I switched from the older 6DJ8, the sound was less noise and more resolution but I did not know if this due to newer tubes or the new tube type. I typically replaced the tubes in the phono stage every year and ALWAYS went with the lowest noise tubes as possible. RAM Labs had a special series of low noise tubes. I then rotated the existing phono tubes into the line stage. It was just too costly and I think unnecessary to replace the entire tube set very often. But the phono stage was very sensitive to tube noise. ARC has come a long long way since then to make lower noise preamps, but only a few models since then have had the musical involvement and magic of the SP-10. Hope this helps a lot.
