15 feet is not a problem for phono box to preamp.. If you had it 50 ft... then balanced would be better...
And yes the cable from the TT to the phono prepreamp should be as short as practical. that signal is really tiny.
One thing though, If you have the 1.5, then you either need the power supply just for the 1.5, or you have a really long cord for the 1.5 from the power supply with your BP 26?
I own an Audio Research PH-2 besides the Bryston 1.5 and BP-26.
The ARC PH-2 is a fully balanced in and all the way through and out. Not very many phono boxes are.