I'm a huge fan of the U5--I've use it all the time ove rthe last year and a half, for electric and upright; I use it for occassional jazz guitar gigs too. It's a great sounding preamp, especially good for deep, controlled bass. But the high end is equallly good, it's not harsh or gritty. It's a just a really pleasing, musical sound. I've used it into the PA many times and the soundmen always comment on the quality of the tone.
With electric five string I most often have it set flat, with the treble cut button engaged, or set at three, which gives a very smooth and fat low mid emphasis. Sometimes I use setting two, a really sharp, deep mid cut, for slap style tones.
With double bass when I used piezo pickups I used setting 4 a lot--it has a little notch in the high mids that cancels some piezo sound. When I was using a realist I sometimes thought it got a little boomy, and in some situations setting five, which has a lot of low end roll off, worked well. For a while I was trying a rev solo pickup, and that REALLY got boomy, so I used setting five a lot. With my guitar, a guild archtop, setting five was often good when I was playing with others, otherwsie flat with the treble cut button engaged worked well
Now that I'm using a shertler dyn-b I like it set flat, or maybe on three, but really, flat seems to be the best. The Schertler is a contact mic, and I step it down from xlr to 1/4 with a shure 195uf line matching transformer. That part's a bit of a pain--I once came to a gig without it and was screwed. But the U5 works really well with the schertler, and a couple of times, as I'm playing live, I've switched from one of the settings to "flat" and it sounds better, and I say why were you overthinking it?
The obvious shortcoming of the U5 would seem to be the limited eq options, but it never really bothers me. I generally run it flat, and if I need some adjustments one of the eq curves gets me close and my fingers do the rest.
Recording (strictly small time home stuff) with it has been mixed--my house wiring is not the greatest and tends to pick up radio. I've had a couple amps pickup radio, and the Avalon does it a lot. I love the tone, but I've been frustrated swapping outlets till I found one that wasn't picking up spanish-speaking evangelists.
By the way, the rest of my live rig is an acoustic image focus SA power amp, and with double bass, an epifani UL 110. With electric, same setup plus an aguilar gs112