Enjoy the show and do try to zero in on a total budget here. Also, try to zero in on which tube sound that you like. For me, there were 2 significant events, shows that I attended which allowed me to listen to many different amps and figure all of that out.
Around the year 2000, I went to a Single Ended Symposium put on by Stereophile magazine here in Philadelphia. Prior to that I'd never heard SE amps and at this time they had returned in popularity. That day I heard SET amps from Dennis Had of Cary, David Berning, Audio Note, Don Garber of FI, and several other brands. I heard 845s, 211s, 300B's and 2A3's. We listened thru 104 db horn speakers, a CAT preamp, Goertz spkr wire and IC's and Sony's new Super CD player or whatever was the new top of the line in CD technology. At the end of the day, the least expensive, smallest powered amp, the 3 wpc, FI 2A3 monoblocks were clearly the best sounding amp in the room. The highest powered amps, the 845's sounded worst - although they were all excellent and I could be happy with all of them, but the 2A3 sounded best to my ears and many others in the room. There were several writers, equipment reviewers in the room from Stereophile, TAS, and Listener magazine. The room was quite large, 40x40 perhaps and 3 wpc filled the room.
Years later, I went to the Vacuum Tube Valley show in Central NJ and heard push pull amps using KT88's, EL34's, 6BQ5's, KT66's and 6BG4's. Again, the best, to me, was the lowest powered amps, the 6BQ5/EL84 revision of the Dyna MKII (18 wpc), and a modded Dyna Stereo 70 which used 6BG4's in PP triode for 12 wpc.
I also got to hear Single Ended Triode amps, one Italian int amp in particular that could operate using 2A3's, 45's, 10's, 300B's, and 50's with the turn of a dial. Each produced a slightly different sonic signature and I concluded again that the lowest powered combination sounded best to my ears.
I also heard digital amps, and digital amps with tube preamps and they were also very very good. A little bit sterile, but the tube preamp warmed them up.
If you are looking at very high efficiency speakers, above 100 db, then the ultra low SET amps would work fine.
I also heard high efficiency single driver speakers from Omega, Lowther and others. The Lowthers I was not a fan of....
As for the Atma-sphere brand you mentioned, I believe it is probably very expensive. A couple of years ago, I picked up an Antique Sound Labs MG-Si15DT for about $400 which runs KT88's in SE pentode and SE triode for 15/5 wpc respectively. I was always curious to hear a KT88 in SE, since I really wasn't a fan of it in PP. It was a great sounding int amp for not much money. ASL also makes a 2A3 amp the Tulip which supposed to be very good as well. Its not a current model, but you might be able to find one used.....That's if you like high efficiency speakers
So, see what you like most, see which tube type and topology you like best be it PP or SE. I never owned or heard an OTL amp, but to me, they are not very practical. They take lots of tubes, they must draw lots of heat and electricity and could be a real hassle. I want something that will run tubes conservatively and will get lots of life from the tube.
I think that a 6BQ5/EL84 based amp whether in PP or SE is a great place to start. It's a great sounding tube and is inexpensive to re-tube. It doesn't give you quite the coolness of a coke bottle styled 2A3 tube but it also way less expensive as well......A long time ago, I had an early Jeff Korneff 6BQ5 SE pentode int amp that was 5 wpc
If you like the low powered SET w/hi efficiency speakers combination, you couldn't do much better than a Korneff 45 if you can find one or a 2A3 amp, IMHO. You can pick up a Korneff in the $1500-2K range
For way less than the price of some of the amps listed above, I could put together an incredible sounding system that could hold its own with gear costing 1000's more......enjoy the show, looking forward to hearing your comments when you return....