A pitch shifter can lower the notes on your guitar when you play them. Usually they have options like Octave, Tone, and Semitone. So you can lower them by either a full octave, a whole tone, or just a couple semi tones.
Only problem is they usually don't sound all that amazing, at least to me, Muddies up the sound sometimes. You can use Drop Bb on 6 string. You need some .13 or higher gauge strings. Dean Markley have special ones you can order that are .15 I think, They feel pretty good that low but you just got to be sure your guitar is setup for it right, heavy strings have heavy tension and I have heard stories about heavy strings warping necks, I havent had it happen to me, But I wouldnt risk it
But like it was said before, best bet is a 7 string. 7 string standard tuning is B Standard (BEADGBE), So you only have to lower a 7 string a half step. Much easier to do.
EDIT
As far as I know, PRS Don't offer any 7 strings, If they do it might be part of the custom shop.
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