Hi Kev --
Naturally, you have some idea about the amount of volume needed, size, weight, cost, etc. Plus, will you ever want the ability to send sound out to an audience. In that case, what size room?
When I need just a little sound to my own ears (in a semi-acoustic situation with the church PA), I use either a BOSE Soundlink Color II or a Yamaha Stagepas 100BTR. Both are battery-powered. Both are pretty clean at low to moderate volume. Neither one has enough output for a hall or big room.
The Soundlink is ultra small, light and inexpensive. I usually have it right next to me and it's just enough to hear myself above acoustic piano, guitar and drums. The church PA does the real work in the hall. Due to my playing position and acoustics, I don't get much sound from the hall itself, so I need a little more "me".
Hope these idea help -- pj