Well the BP-26 and BP 1.5 have a lot of gain.
ANY phono box is going to have a trace of low level hiss when turned way up. (depending on speakers etc..)
IF the hiss can be heard at normal listening levels, from your normal seating distance, then it may be more than usual.
(Standing next to the speakers with ear two inches from tweeters, or with the volume control maxed out is cheating!)
PS: I am a high frequency nutcase. That is the #1 area a system has to be perfect for me to even sit and listen.
Anyway, The problem IMO guessing seems to be a ground with TT.
Other possible issues are the Bryston preamp power supply is too close to the Bryston phono box. Or the connecting cord from the power supply is tangled up with a regular powercord?
My Bryston BP 1.5 is pretty quiet from my Rega P5 with Benz Glider.
Also, I found the interconnect from the BP 1.5 to the BP-26 to be very critical.
I had a Kimber KCAG, but it was clearly too bright. Switching to a custom made RCA ((Vampire Tiffany style RCA) set using old (1980ish?)Mark Levinson wire really was the perfect cable. (the closest premade cable would be a midpriced Cardas IMO)
Finally, I wonder if the OP uses a power conditioner. The A/C in his place may be not so great?
I use and swear by big conditioners. Using a Furman REF20i for all my analog, and a big PS Audio P600 for the digital stuff.
Last, sorry the op's BP1.5 is not up to what he hoped for.
The dealer is the guy who should be working with the op at no charge over the issue. And if the op is not happy, well the dealer should work to make him happy. (this is assuming the op bought the BP1.5 new from a local dealer.)
