I will answer without seeming a Bryston fan.
The Bryston is NOT a warm amplifier at all.
The Bryston is going to be neutral. and really pass through what you put in. It is also very clear, no fuzzy additions. Though a power conditioner would enhance that some.
I am still impressed how nice my Bryston amp, preamp, and phono box sound. And admit I have worked on various free tweaks to get the last possible bit out of my Bryston stuff. Mostly concerning power conditioning and cords etc.
So if you BUY a warm preamp, say a Conrad Johnson, then your sound will be like that preamp. Since it has a pretty clear know warm sound.
I myself bought a VAC Standard four tube preamp to use as a fancy tube buffer for the digital stuff.
(I kept my old cheaper digital gear when i bought the Bryston and the tubes 'fixed' my digital 'problem' perfectly.)
So if you actually want warm sound, then with the Bryston, you can buy some preamp with that coloration, and know the Bryston amp will not add any further changes to the sound.
I have No special loyalty to Bryston, and if something bad was afoot, i would 'diss Bryston in a heartbeat. (aka my Bryston DAC experience) so I have no qualms saying i am giving an honest answer to the op.
This may be a little rude to Bryston fan types, and all. but the guy is gonna get an honest answer from me.