The U5 is a Di box with limited tone shaping options. You can use it as a DI or as a preamp or as both at the same time. Avalon makes high end recording gear. The idea of the U5 was to build an extremely high quality DI--class A, high voltage high current--with very simple controls. I play about 4-8 gigs a month, mostly electric bar band gigs and upright jazz gigs
It's an astonishingly good sounding piece of gear. It gives you really deep, rich, warm lows and clear, sweet highs with no unpleasant gritty clanky bite. The tone coming out of the thing is both "fat" in the best sense and also very hi-fi--but not in a bad way. I have always shied away from "hi fi" figuring I play bass, not piano or guitar, and I don't want all that range. The U5 has none of the things I disliked about "hi-fi" (sterile tone, irritating highs, boomy lows, weak midrange) but has all the range. As a result the bass sounds great--sweet, whole, musical. I've used ampegs, SWR, and sansamp preamps mostly. The Ampeg B15, to me, is about as good as you can get but it's a 30 watt amp. The U5 is not as "tubey" as the B15--no tube compression--but it's not as saggy either, and the sound is equally pleasing. You don't get that irritating set-jaw ear fatgue thing with this box--it just plain sounds good. because it's so sensitive, it picks up your playing style much more effectively--variations in touch are clearer right way, as are changes in the tone knob. Rolling back the treble doesn't make the sound muddy, it just cuts some treble--you still hear yourself, the tone control just has more precision. If you've ever plugged your bass into a really good studio preamp, you know the sound I'm talking about.
I've done about ten gigs with the box, using both upright bass and a passive four string j-bass. It's a revelation. A lot of amps/preamps offer you tons of eq options--active tone controls that let you boost and cut your way to sounding like anything. The downside, I think, is that they tend to mask your playing a little. I really liked the sansamp RBI--versatile, cheap, functional--but it's hard to see going back to it. I can't imitate an SVT with the avalon, but I can get a gorgeous rich bass sound that reflects what my fingers are doing much much better. It sounds deep without mud, authoritative and articulate. I play more or less old school fingerstyle, with flatwounds and a little bit of mute. An ampeg b15 is my ideal amp. The hi-fi thing should bug me, but with the Avalon it doesn't.
The six preset tone filters give you some very nice settings--most are pretty subtle. The last two radically cut the bass, the second cuts the mid-mirange sharply. I don't use them much--it's mostly just the eq filters off and I eq it with my fingers. I usually have the hi-pass switch on, to cut high string noise. Otherwise, it's just straight into the pa or a power amp, or both at the same time--you can send the signal to the xlr line out AND to a 1/4 out whch you can then feed to a power amp, for a monitor or to support the pa. Or you ca run the xlr line out into a poweramp
You don't believe me--you're thinking how can it be any good without a bunch of tone knobs? Well I don't know, but I DO know I don't miss the knob twiddling at all, the bands LOVE the tone, and I spend more time actually playing--the whole experience of bass is just more musical.
Downsides: it's heavy for what it is, and clunky. You can rack mount it, but it takes two spaces and the rack mount adaptors are pricey. Right now I have it in a little skb case--more to carry! It's got limited eq options--yeah, I said i didn't care, and don't, but facts are facts. So far i have not missed the tone controls one bit
Simplicity--a great sound, simple controls, concentrate on playing music instead of playing the amp and imitating someone else