the general difference between a pitch shifter (moves in semitone or whole tone steps) and a digital octave pedal is an octave doubles or halves the frequency without regard to frequencies coming in. take a look at this wikipedia link

Now, a guitar’s low E is 82.41hz for the fundamental. Each octave down divides that in two – so , -1 octave is 41.25hz, -2 octaves is 20.77hz. The lower limit of human hearing in otherwise silent/anechoic chamber situations is 14hz; the generally accepted ‘lowest frequency you can hear’ is 20hz (also generally around the lowest frequencies most amplifiers will put out).

Now, with digital effects, the low frequencies are left wholly intact – I mean, the Digital/Analog converters aren’t going to screw with your lows (your highs maybe but that’s a whole different matter that you can read about at this link) – but the amplifier or preamp sections might trim it, and it’s doubtful the speakers will reproduce it.

Now let’s look at your droptune B -15vb scenario. B1 is 61.74hz so B0 would be 30.87hz and B-1 would be 15.44hz. That’s no longer audible. That’s no longer sonic frequencies – that is kinetic frequencies. That is rumble. infrasound. your speakers will not be able to accurately reproduce it. True, you’ll get the harmonics showing through, but the fundamental note will not be there.

Also the PS-5 was a terrible pitch shifter – it and the Hr-2 (another turd) replaced in the boss catalog one of the best pedals ever made, the PS-3. The Micropog or Pog2 will get you what you need.