Tube Amp for Green Mountain Audio Callisto

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!

thanks for all the excellent responses.

using a radio shack SPL meter, i turned the music on as loud as i care to listen with the following results:

~88-90db from the listening position (the big peaks sometimes hit 95), which correllated to ~90-92db 1m from the driver (big peaks hitting 97).

on paper it would seem that i could get by with around 8 watts, as that would equate to 99db.

i also called Roy - he so amicable - and he confirmed that with as little as 15 watts, the callistos will sound great.

i was thinking an atma-sphere S-30 would not have enough juice to get my speakers singing, looks like i was wrong. i'll also look into the 300B, Jolida and Cary options.

thanks for all the suggestions!

Blue Circle gear has great synergy with GMA speakers. Disclosure: I am a Blue Circle and GMA retailer, Departure Audio. The DAR and FtTH ate hybrid integrated amps in the Blue Circle line. The new GDC integrated, while not a tube piece, is a terrific value at less than 2K and also a great match with GMA speakers. All of these have more than enough power to get everything out of the Callisto's. Blue Circle is one of the few lines GMA enthusiastically endorses with their speakers. Roy and I will be demonstrating these systems at some upcoming shows.