Reversing phase is not possible on the BP17.
Is this something we should have on the BP18?
IMO, it's a toss up. I have it on my BP-26, but seldom use it, except on a few poorly-recorded 16/44.1 CDs
So far, I haven't missed it on my BP17c preamp.
It might be my choice of CDs, since I only play classical and organ CDs. No digital, hard rock, heavy metal performances.
Others, specifically Redbook and Elizabeth can honestly disagree, as they both do use it.
BTW: James, you've said the BP18 will be a combination of the 26 and the DAC-3. Since the 26 has Polarity reverse, inclusion shouldn't be an issue.