I had the BP6 as well and it’s a very neutral preamp but when paired with a Bryston 2B SST I found what you’re finding in that it was a little thin sounding. But when I paired the BP6 with my McCormack DNA-0.5 RevA amp it was fuller and more natural sounding and much more balanced IMHO. I’d recommend taking a look at this DNA-225 that’s a good value and should flesh things out very nicely. You could also look at your DAC as upgrading to one of the newer R2R DACs might be more to your sonic preferences. Hope this helps, and best of luck.
https://www.usaudiomart.com/details/649982529-mccormack-dna-225-power-amplifier-works-perfectly/