tuning to Ab Eb Bb Db Gb Bb /// Ab Db Ab Db Gb Bb

why don't you just play a six string bass?

To tune down to these unusual tunings you could do one of two things. First you could buy a chromatic tuner which would allow you to tune to any note.

Or two you could tune one of the strings with the string next to it and work your way across the strings. The notes all go in the same order on every string Ab,A,Bb,B,C,Db,D,Eb,E,F,Gb,G,Ab....(and repeat) Just tune the string to a note that you already know and figure out which notes are at which fret. Then tune the other string to that. Then using the string that you just tuned tune all of the other strings.

Of course it sounds like with these notes that you have stretched your strings to impossible limits. It seems to me that you might want tuning where each of the six strings is dropped down one tone. This would go from standard tuning E,A,D,G,B,E to Eb,Ab,Db,Gb,Bb,Eb. This seems more like what you want since you wouldn't want to warp the strings so much to get some of these notes.

Anthony