“Tannoy DCs sound best with amplifiers with medium output impedance /damping factor.That generally means push pull valve amps of more than 15 watts”
Although Tannoys usually rate in the 93-94 db rage, they don’t respond well to lower power SET amps. Heck my 15 watt Quad ii are anemic with the Tannoys. My Frankenstein 300b are good for background music an not much else. My ASL845 amps are the starting level where the it started to sound very good, and it controlled the woofer well. In the end, my Churchill’s benefit from more power still. My Luxman 509x delivers a very warm sound without any horn brightness. And my Atma-sphere M60 is the sweet spot with enough power, and being OTL, by removing the output tranny rivals the magic of SET.
For me there is more magic in the Atmosphere / Tannoy combo than my 94db Coincident Total Eclipse (TE) and 300b Frankenstein, which was a formidable combination in its own right where the 300b is more than enough to drive the TE speakers which present a much easier load at 10+ ohms.
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. Buy used, and if you don’t like it, you can swap it own without any relative loss.
I’ve also had PassXA25, ASL Hurricanes, Coincident Dragons PP211, and the Atma-sphere was the best combo for my Tannoy situation.YMMV
All the best in finding your utopia, and enjoy the ride!