I think that I may have found an answer for the bluegrass bassist playing an electric bass and needing amplification that is also portable, allowing to play where there is no power.
A few weeks ago, I won an eBay bid on a Behringer Europort HPA40 battery-powered PA system. Back when I was playing in electric bands, I remember the Behringer brand as a decent source for low-cost solid-state amplification, especially with bass guitar, keyboard, and PA system amplification. Phasing out of my electric guitars and moving toward acoustic instruments, I had forgotten about the brand and was a wee bit shocked that they were still around when I saw this item on eBay.
I got my model used, so it didn’t come with accessories. Brand new, it retails for about $150 at most of the online music stores. It is about the size of a toaster, is shaped like a torpedo head, and runs on a rechargeable battery (or AC adapter). The specs say that it has 40 watts of power through a 5-inch speaker. I assume that the 40 watts is peak, and probably runs about 10-15 watts regulated, which is still great for a battery-powered amp.
When I received it, I plugged my acoustic bass guitar into it and was surprised at the amount of volume that it kicked out. The speaker was more bass responsive than my Pignose, which makes it better for porch jams when some bottom end needs to cut through.
The controls are basic. Volume control (no EQ or tone controls), push-button power switch, 1/4-inch mic input, 3.5-mm line input, a USB input for optional Behringer wireless microphone, and an input jack for AC adapter/charger. LEDs light up for power as well as battery life. Accessories include a power supply, a dynamic microphone with a 3-foot coiled mic cord, and a carrying strap (which can be stored in the rear of the amp where the rechargeable battery sits). Plugging the bass guitar into the mic input gives a bit of distortion, so it would be wise to cut some volume from the guitar controls. If using the line input, you would need an adapter.
To be honest, this is the best battery-operated (albeit a rechargeable battery and not disposable ones) portable amp that I have seen in a while for the price. It has enough volume, and the specs state that it will go 8 hours on a full charge. The 5-inch speaker has enough low-end response to make the bass guitar sound like a bass guitar. While Behringer seems to have developed this as a mini PA system for a presenter to use in a small conference room, it does the job as a music amplifier. I also see it being used perhaps as a small outdoor PA by a bluegrass combo working a single microphone (NOTE: this doesn’t have phantom power), maybe not with the supplied dynamic cardioid mic but possible with an omnidirectional one. This is a great busking PA.
While $150 may seem pricy for a small unit, remember its portability. Also, you may find used or refurbished ones on eBay or your local Craigslist. Make sure that those versions work, as this unit does not look like an easy repair. If you get one, treat her kind and she will be good to you.
I also see that Behringer markets a similar model, MPA 30BT, which has a larger speaker, runs for 20 hours on a charge, and has a built-in 2-channel mixer for about $180 with no accessories.
Chew on it and comment.