Is it possible to strum a 5-String bass with the B-string tuned down to A (or B flat)? I.e in a Drop-A tuning? I set the PolyTune 2 to Drop-D mode but it does not work.
No, unfortunately this does not work. It is possible to tune a 5-string bass in standard tuning with the polyphonic mode, but this does not work if you have the B dropped to a low A. Unfortunately, using Drop-D mode does not help this either.
The only way to get around this is to tune the G, D, A, and E strings in polyphonic mode and then tune the B-string alone using the strobe or needle mode. Please see page 7 (9 on .pdf) on the user manual for more details.