A friend of mine has an 8 string built custom for him by a local luthier - it's a copy of the meshuggah ibanez, so basically an RG with a 30" scale length ( :eek: ) and the strings are so close together you almost have to play the damn thing like a twelve string...

He tunes it F, Bb, Eb, Ab, Db, Gb, Bb, eb as I believe meshuggah do - from experience, I'd say 7 is the usable limit. If you wanna go lower, play bass.

IMO it isn't the low tone that makes a heavy sound, it's just a cheap way to fake it.... It would be borderline impossible to play many chords, and for lead it's limited as well, so I don't see an advantage...