To the OP mmcgill, if you like the purity of SET, and your down the the two choices only, I think the GM70 in the NAT audio single HPS v2 has got the minimum current and can drive the Tannoy to sing well, and will really sound sweet. As usual it about personal preference, synergy, and your room. You need to find what works best for you.
Good comments and suggestions. That NAT has enough quality power I believe for these Tannoys. And, the OP has heard this NAT with Tannoys and found this pairing sounds "incredible". I can see where the Atma-sphere M60s would be a very good option, no doubt. I also agree that an 8 watt 300b SET would not be the best choice with these speakers.
Charles