SET for Orchestral , Big Band etc.

Depends on design as regards the sound of a 300B amp. But as everyone knows, the speakers must be an easy load - higher and steady impendance (6+ min. without going much over 11-12 ohms), efficient (87db at least), and an easy phase angle through the crossover (which also contributes to impedance). Get these three things in a speaker (plus musicality) and the 300B should put an orchestra in your room. I use a room of almost the same size, 11 x 13.5 x 8, and currently use an Air Tight 300B to damn good effect.

Phase angle is a by product of the crossover design. It's a good way to measure some of the difficulty or ease the amp will have driving the speakers. A benign phase angle will reflect a stable impedance load and therefore an amp has to work less hard to get the speakers up and running.

Having a stable (not to low and not too high and not wild swings) impendance load is just as important as efficiency. A speaker can show 91db eff. but have a wide impedance swing across the frequency spectrum (say, 3.5 ohms to 20 ohms) and therefore require a great deal of current to get going. Some Wilson Audio designs have fit this description.

Merlin TSMs are kind of the opposite - efficiency of 88-89db but a very stable and quite narrow impedance band and therefore quite easy to drive. I had a pair of them which the 300B amp drove just fine in my small room. Same thing for two ProAc models. I now have Harbeth C7s which, while wonderful speakers, need some current to sound their best. The Air Tight does not work well with them so I have a Plinius 50 watter which does superbly. Soliloquy speakers also are generally easy to drive but their sound is not to my taste.

Best thing to do is search this website for discussions about SET friendly speakers (there've been many of them). I would also always contact the manufacturer or importer of
whatever you are considering.

Also, 300B tubes are about 3 times as powerful as the 2A3 so you will need a speaker of at least 95db with and easy impedance load with the 2A3s. This substantially narrows your list. Horn speakers are an option but, man, do they have their own sound and you would absolutely have to try them in your room first. I've always found horns do best in med or larger rooms as I think you need to sit in the med or far field to not get that "honk-glare" effect.

I personally have never heard one that didn't have some of that stuff. I personally like a more relaxed, less tense presentation. But, this is all personal taste. I have a friend who loves his horn loaded Lowthers but I can't stay in the room for more than 2 minutes.

I still say you'd better audition any horn speakers in your home. Almost a must for any new purchase but an absolute must with horns.