Unfortunately, the needs of acoustic and electric on stage are strikingly different as a rule. However, you can get around that a little by using a wicked clean acoustic amp and an amp modeler. For instance, one combo that works fairly well is an SWR Strawberry Blonde or California Blonde used with a Pod, Vamp, or Vox Tonelab. The acoustic goes straight into the acoustic amp, and the electric goes through the amp modeler, with the cab model disabled, then into the acoustic amp. Tube amps rarely work well for acoustic, as they tend to introduce controlled distortion which is often good for electric and bad for acoustic. For small solo gigs, I often jack the acoustic straight into a small PA, and the electric goes through a VAMP II into the PA. I use a Fender PD250 Passport for this. Perhaps when viewed as a PA, the little Fender is pretty weak, but if you consider it as a 250 watt
acoustic guitar amp, it produces plenty of power.
If you really want that kick-ass, cranked-up tube-based combo amp tone, you will need 2 amps. For pure onstage acoustic sound, I've heard no acoustic amp that beats a Strawberry Blonde, although my Passport comes pretty close.-Richie