PREAMPLIFIER UPGRADE QUESTION

If I had your system, my only quandary would be to decide between the Benchmark LA4 and the Bryston BP-19. I currently have for amplification the Bryston 14B-SST2 and the 4B3 which I use with the LA4, the Bryston BP-26, and the Crane Song Avocet IIA. I am, however, familiar, with the BP-19 and have one on the way.

What you'll get with either the BP-19 or the LA4 is transparency, striking dynamic range, ultra-low distortion, and reproduction that is neither "bright" nor "warm," both of which are forms of distortion.

However, if, in the end, you opt for a tubed preamp, make sure it has an output impedance that is sufficiently low to drive the (relatively) low-impedance balanced inputs of the 4B3 amp without compromising dynamics or frequency response.

But if there's any uncertainty on this point, it's best to contact the manufacturer of the preamp for guidance.

Seems like the Bryston is an obvious choice since you know you like that sound, there should be some synergy between same brand products, and advantages staying with one dealer if you trust him. Plus, Brystons are built like a tank and I highly doubt anybody trying to sell you something else will match the warranty. If possible listen to all your choices and buy what you really like-if you don't like any of them, keep looking. FWIW (not much because what sound we like is personal taste) the Brystons I've heard are transparent, but not overly dry. I like a natural  sound that is not unduly colored and if that's your taste there is a lot to like about Bryston, it seems. If you want a more colored sound, your other choices might be preferred.

Good luck!