Sam,
With your size room and it opening to other areas, and the sensitivity of your speakers, typical SET single power-tube amps (8-12 watts) might be enough, but as always.....it depends.
Suggestion... since you are looking at a possible $3500 investment:
Get one of the less expensive models first (Jared 300b for $500).
See if that drives your speakers to adequate levels without strain/clipping. If so, you're home free and have many choices with higher-quality SET amps putting out at least 8w. Then get the premium amp that you want. (you'll get several recommendations here, I'm sure!)
If the power is NOT adequate, you'll obviously need to UP the power, or change speakers if you like the SET sound. (It IS in a category of it's own!)
If you decide you just gotta have that beautiful SET sound, Sell the cheaper amp at not much, to no loss and be happy!
If you should happen to have them both at the same time, a/b compare the cheaper one to the more expensive.You might be tempted to keep it for/in a 2nd system.
Also, depending on your location, you might find a fellow enthusiast that would let you try their SET amp in your system. A bunch of us bribe pretty easily with the offer to share a decent bottle of wine! :-)
Good luck and Good listening!