I had the good fortune to pick of of these little battery-powered (or optional AC-adapter powered) monitors on eBay a little while ago, and it's quickly becoming a favorite portable theremin amp for personal monitoring:
It's called the Roland Mobile AC, and its a dense and rugged little unit.
It's not going to overwhelm you with bass, but it has dual speakers, five inputs (two 1/4" for guitar and mic, stereo RCA, and one stereo audio 1/8") with individual level controls for audio, mic, and guitar. The other end has the power switch, adapter jack, and a stereo headphone output. And it has a variable reverb and switchable chorus and wide effects built in. The clarity and presence with a theremin is great for monitoring, and I find it really handy to have something that is known to be ground-isolated when testing theremin noise issues.
It uses AA batteries, supposedly good for 15 hours.
I had one of these is a shopping cart for about a week while I second-guessed it, but just before I pushed the Buy button one popped up on eBay for $65 with free shipping (about half the new price). It was a new demo that had been set aside because it was supposedly intermittent (a potential easy fix), but if it ever had a problem it hasn't shown up yet.
Did I save any money in the end? Not really. I liked it so much that I bought the second one that had been sitting in my cart the whole time to keep on my bench. Lately I've been using it as a pitch preview monitor too. I haven't tried it with an acoustic guitar but it is often suggested for use as a lightweight busking amp. It even has a threaded insert on the bottom to mount it up on a mic stand.
Entirely decent for the price, IMO.