Tube Amp for Green Mountain Audio Callisto

Do you have a decibel meter (like a Radio Shack)?

If you do, turn up the volume to your normal listening level at the listening position, then measure the decibel level from a distance of 1 meter directly in front of each speaker (left then right, or vice versa).

This will provide you with the decibel level at 1 meter when the volume is at your normal listening level. Try to measure a couple of volume peaks as well to get an idea of decibel level at loud points in the music.

For your speakers, 1 watt of power produces 90dB measured at 1 meter. Each doubling of power will produce an additional 3dB. So...2 watts will produce 93dB, 4 watts will produce 96dB, 8 watts will produce 99dB and so forth.

Using this info, you can determine how much power you will require to produce the volumes you prefer.

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!