Much of the problem has to do with the maximum headroom allowable on small mixers. Some of them are capable of being attenuated down to a level where they can handle typical audio levels coming off of a synthesizer. I've done this a few times with my Mackie 1202...but we're talking about a 25-year old original 1202, which was a bit of a different beast than the present-day Mackies. I've read some accounts of how the Allen & Heath ZED series is also capable of dealing with the high incoming levels, also.
Another alternative to an in-cab performance mixer that would allow you to route directly to an outboard mixer would be any of the various output modules. Since these are designed to step down the levels to line-level, a bank of several of these would also make for a cost-effective and space-saving option. Ladik has several options here that are worth examining, such as their P-530 dual output module which offers attentuation plus 1/4" outs that should interface easily with any outboard mixer you can think of. In 16 hp, you could have trimmable eight line outs for only $160. Frankly, that seems like the best option, leaving you free to use whichever mixer suits your performing needs.