I've looked into sub-bass tunings because I'm a DADGAD player who uses a lot of partial capos which results in quite a number of odd "open" or at least "effective" tunings. I decided that I would absolutely have to have a full set of sharping levers. Working high to low (down, pitch-wise) from the low D of DADGAD the sub-basses, with sharping levers, would cover C/C#, Bb/B, G#/A, F#/G, E/F and D/Eb. Sharping levers would make it so much easier than having to re-tune those strings to suit each tuning, especially when I go out of my way to NOT have to tune the DADGAD strings!
One recent innovation that I've been intrigued by is the
Pitch-Key. This could be an elegant solution to re-tuning the sub-basses. It would need a nut post installed for each sub-bass string but not require any other modification to the instrument. Problem is that they're not making them for anything larger than a .054 string...
Phil