Oh yeah, it's definitely back bow.
Trying to shorten the story, it started with a wave at the 12th fret. Straight from 12-20, slight hump at 12, and a normal amount of relief between 12th and 1st frets. Moderately annoying. Been scouring through Rick forums and talking with people there who recommended removing the rods, straightening them (some are apparently curved, as mine were considerably, from the factory while others are straight), and resinstalling. Did just that and the odd wave was gone.
Now there's considerable back bow. Using a straight edge on the frets, the straight edge is on frets 1-12 and starts to rise off the frets until about 2mm off at the 20th fret. I've re-curved the rods, turned them in the channels 180 degrees, kept the rod nuts completely loose, tried heavier strings (.045-.130, I usually use .032-.100), strung it with only one rod in. Nothing has cured the back bow save for the heavier strings, and they only do enough to make it playable for the weekend, not where I'd really like it set. I've always had Ricks set dead straight with very light strings and had no problems.
This leaves me down to heating the neck as the last thing I can find to try, unless there's a better idea here. I think the knowledge level is considerably higher here on how to repair something than a forum of mostly players, which is why I thought I'd throw out the question.
One other note, this was once a 4 string Rick and was converted to a 5 stringer. I don't think that should be an issue because I'm under the impression Rick used the exact same neck configuration for the 5 and 4 stringers. The guy that converts them has done numerous Ricks with no ill effects (he and others who have his basses say). And he clearly remembers this one already having that odd wave prior to conversion for the person from whom I bought it. Plus, I would think if the 5 strings added too much tension, the back bow should be cured by that, should it not?
Kind of at my wits end on this and wondering if neck heating described in this forum will work on a finished neck or if there's a better idea.
Thank you very much.