Tube Amp for Green Mountain Audio Callisto

Why not ask Roy at GMA what his minimum power recommendation is?

With requirements like that you could get by happily with a 3W 2A3 SET amp. You would lose a little bit in dynamics, though.

With an 8W 300B, you'd have plenty of dynamics. (SETs, with their class-A operation and frequently overbuilt power supplies, and ability to swing much higher voltage than their wattage rating suggests for brief periods, usually have excellent dynamics within their power capabilities.)

Roy's speakers are an extremely easy load and SET-friendly despite the moderate sensitivity rating.

Scott,
Tho' I not owned the exact speaker you own, I did own the Green Mtn C1.5i 3-way floor-stander. My speaker was a 6-Ohm load just as benign as your stand-mount. I was quite successful driving my floor-standers using 2 very modest amps - the Jolida 502A, which was/is 60W/ch in ultra-linear mode & the AES/Cary Sixpac, which was/is also 60W/ch (because I had an upgraded power xformer) but this time in triode mode using EL34.
Both tube amps created beautiful, seductive music. My personal favourite was the AES/Cary Sixpac.
I believe that you could try either tube amp for not much money (considering the times). FWIW. Good luck!