Had a look for some thicker short scale strings, given the popularity of the short scales you'd think there'd be a few offerings, but alas not so far, DR seem to only do the one set. Moot point though for Matt, he plays the sub now and the Jag is the spare. Served it's purpose well though, longer scales are harder to start, I'm quite blown out by his progress, and the sub sounds so good.
I'll tell him about the GHS 50's that sounds like the perfect gauge, though I'm not a fan of ground wounds (but who knows, he may be if he tries them?) or even worse flat wounds, loose all the nice tops...
A bass player in another band I was in bought a Mexican Fender Jazz, sounded lovely, almost like a piano (ala the OX Entwistle), then he got me to put a set of flat wounds on it (I offered to set it up for him), Fu$$ed it completely, sounded dead although he liked it (easy to play), oh I'll just turn the treb up.. sorry doesn't work like that..
So your up bow, is the truss rod not working ? (had more than my fill of that over time...)