I’ll look into Bryston BP26, but I’ve found past Bryston components on the sterile side, but it’s been decades.
Bryston’s “sound” has evolved quite a bit over the years, and while I agree their older amps were a bit thin and sterile sounding I wouldn’t characterize their later offerings that way. As far as their preamps, I’d not call them sterile but rather neutral and very transparent. If the BP26 is fed something warm it’ll simply pass that on and likewise with something sterile. While it won’t add anything of its own to the sonic signature, its complete lack of noise or any kind of veil whatsoever lends a sense of openness and clarity that results in excellent imaging and a vast and unfettered 3D soundstage if it’s in the recording. It was my understanding these were the characteristics that are important to you and hence my recommendation.
As far as the tube preamps, I’ve not heard any of them but from the reviews I’ve read they are among the least colored or veiled tube preamps available, which is again why I thought they’d be good recommendations and I’d encourage you to read their respective reviews if you haven’t already (BTW I’d also recommend the Atmasphere MP3 but since your budget allows for the MP1 I mentioned it instead and maybe @atmasphere can elaborate on their differences as well as the bass concerns you have from reading the MP1 reviews). Anyway, hope this helps and FWIW.