I bought a bass once that was some off brand lowbuck bass, but it was beautiful, neck-through and the wood was an exact match for a Kawai guitar that I had picked up at a pawn shop (the pawn shop sold the Kawai to me for $120 after I convinced them that the electronics were fried...then I took it home and put in a new battery...that Kawai guitar had more knobs and tones than an early Alembic, I think Kawai had peeked inside an Alembic before they made that one, but that's another story).
Anyway, this bass developed a buzz problem that would not go away, and it was obvious that it had developed a slight backwards(!) bend, in the lower (towards the head) 1/3 of the neck.
I took it to Valley Arts Guitar and they said it needed a heat bend and that they would also have to do some refinishing to the neck after the heat bend.
I really didn't want to spend much on a lowbuck bass, so they said they could try a cold bend where they clamp the neck in a jig and let it sit for a few months, but they were pretty sure it wouldn't last. I never really played the bass much, I just liked the look of it sitting on stage next to its little sister guitar, so I didn't mind leaving it there for months, and the cold bend was so cheap I figured it was worth a try. I went ahead and had the cold bend done.
It was there for 4 or 5 months as I recall, and the neck was fine for maybe 6 months after that, but the warp came back.
In the meantime, I had met another luthier who handmade classical guitars and talked to him about it, and he said that a heat bend usually worked fine, but sometimes it didn't work 100% (maybe he was too gentle?), and occasionally, it had to be done more than once, before all of the warp was finally eliminated.
After the cold bend didn't last, I took it to this luthier to do the heat bend. Once again it was fine for a while (a year or so I think) but once again the warp came back, but not nearly as much as it was before. I would say that the warp which returned after the heat bend was much less than half of the original warp, so the heat bend had done -some- good.
Ultimately, rather than do another heat bend, I had him remove the fingerboard and shave the neck a bit and then all was well. Again, some minor refinishing was required. One of the reasons that I opted for the neck shave, was that the bass had a maple fingerboard, but the Kawai guitar had rosewood. Since the fingerboard had to come off for the neck shaving anyway, I had the fingerboard replaced with rosewood. At that time I also installed better pickups, tuning machines, electronics and a Badass bridge. I figured that since I had put it through so much pain, it deserved it.
It was really a very nicely behaved bass after that and I used it a lot for practicing. Unfortunately, both it and the Kawai were stolen one night by a burgler who climbed in my kitchen window. I never got either one back.
That's my one and only experience with a heat bend, and unfortunately, in that case it didn't work 100%.
