Why mess with a mixer when you can put this little doodad in your gig-bag? It seems to do precisely what you want without having to hobble together an awkward system.
1. I'm afraid that these solutions are really junky build-wise. That I'd break them within six months, even when used carefully.
2. They're battery powered. I'm not sure if that's a plus or a minus, but it's one more battery I have to worry about in my life.
3. They're clearly designed to plug directly into an instrument and provide their own amp emulations and effects. I don't want their cheap effects and emulations. I really, really want to use my own pedal board for these things--just like I intend to do live.
4. I worry that my active bass and my pedal board might provide too hot a signal for these units (which as I said are designed to plug directly into a bass). If that's the case, they'll distort in unpleasant ways. I've seen some people say they don't do that, but I've seen other people say they do.
5. It's not like they're saving me any money. I haven't seen one yet that's significantly cheaper than getting a mixer like the Behringer one I've picked out.
6. I have seen one unit that seems to be everything that these little pocket units aren't: that British import someone posted a few pages back. Metal casing, two sturdy looking knobs. No frills. It inspired confidence. Problem is, it costs $150. That's kind of harsh for $3 worth of circuitry and $2 worth of knobs and jacks. My inner cheapskate cries foul. Your mileage may vary.
7. In spite of what they advertise, I believe I'd still need an adapter to plug it into my Ibanez SR300. I don't think it's 90 degree swivel plug thingy can accommodate the input jack on these bases. So I'd still be in for an adapter, detracting from its supposedly simple design.