I have a room similar in size to yours. I'm using ProAc Tablette 50 Signatures with a tube pre and SS amp like you, though mine are the Blue Circle BC21 and BC22. Those little ProAcs sound very big and very satisfying in my 12 x 13 room. Your 4B-ST is probably way more than you'll ever need, but the musical nature of the big Bryston I feel will compliment most any speaker you end up choosing.
Although you say you can do without low bass, I've found I have to run my Tab 50s with a sub for best results. I think the monitors disappear better in my room when using a sub, and the extra low extension adds impact and realism. Maybe a slightly bigger monitor like the ProAc Response 2 would give you adequate bass without needing a sub. There have been some popping up used on Audiogon recently.
Godd luck.
Although you say you can do without low bass, I've found I have to run my Tab 50s with a sub for best results. I think the monitors disappear better in my room when using a sub, and the extra low extension adds impact and realism. Maybe a slightly bigger monitor like the ProAc Response 2 would give you adequate bass without needing a sub. There have been some popping up used on Audiogon recently.
Godd luck.