I've been holding off on my reply to the OP question because I didn't want to rock the amp-power boat, but I'll speak up since there seems to be some consensus, just to really address some concepts related to amps and bluegrass.
While most of the replies here are great for general DB use with jazz and other genre, bluegrass is a somewhat different animal because of the one thing that really sets it apart: Acoustic Jamming...
If you're wanting an amp that you can use to make sure you are heard over banjos, fiddles and other loud acoustic stringed instruments in bluegrass jams, you really want one that is not going to make every one else in the jam angry. The most common rule of thumb in strict bluegrass jams is no amps, no drums, no electric instruments, and you can add to that, no sounding electric. So for the double bass in bluegrass jamming, the safest amp to use is no amp. IF you feel that you have to use an amp to be heard in a bluegrass jam, complaining to the banjo, fiddle and other loud acoustic instrument players that they are playing too loud is fully acceptable, especially in high level jams where this kind of problem shouldn't happen in the first place because experienced bluegrass instrument players should know to use appropriately sensitive volume.
That said, less high-level bluegrass jams do happen, and less experienced bluegrass instrument players often do play too loud. A good, not-too-subtle sign to them that they are doing so is that you have to start using your amp with your double bass, and often even the threat to do so is enough to get people to quiet down.
It's worth mentioning that because of that strong preference for pure acoustic sound, unless they have experience from other genre, most bluegrass instrument players have no experinece with setting up and moderating an amp or a PA properly for use with bluegrass. As a result, those bluegrass jams that do allow amps usually find themselves having amp-wars because players will continually raise their volume until they are loud enough to hear themselves over everyone else. This is a very nasty thing.
On stage is a completely different matter, but still at least one of those rules of thumb still applies: no sounding electric. The best way to avoid this on the festival stage is to use mics, not piezos, and when at all possible, leave the EQ and other tone settings to the bluegrass-experienced FOH. If you're doing your own sound system for an event, a smaller situation or venue, again, the best bet is to use mics instead of piezos, and also hire someone as a regular part of your band to understand how you want to sound, to help schlep and setup, and to run your sound system while you're playing.
Our main band (really in the context of ownership, my wife and I) owns two separate complete sound systems:
One is a fully blown wired PA with multiple PA-quality 550W speakers, full 20 channel mixer, good quality mics, stands, and all the wiring that is required to make it work. And we have a regular sound guy who helps schlep and setup, and who runs the sound system while we play. If we get a gig where this PA system is required we charge significantly extra for use of this hardware and to pay our sound guy. We'll use this with weddings or other special events that will pay well enough to use it, and it works out very nicely.
The other sound system is comprised of the same mics and stands, the necessary wiring, but instead of the mixer and PA speakers we use three Roland Cube Street amps which each support two separately EQ, voicing and tone-controlled mic inputs, and we use a Roland Micro Cube Bass RX amp for my DB (which provides the same settings and also can accomodate a mic). These are all battery operated so we don't have to plug in, and they all offer a "clear" or "mic" voicing so we can avoid sounding electric. We'll use this sound system when we aren't getting paid for our own sound but want it for our own convenience, and/or when playing gigs outside or in venues that don't have electric power. It will serve audiences of up to around 50 people inside or out, and we can set it up and leave it through the whole gig. The key to successfully using small speakers is to get them up off the ground where they are no longer shooting sound into peoples' feet and legs.
The Roland Micro Cube Bass RX is my go-to bass amp, I can use it as a personal monitor while also feeding an FOH, or I can use it as a stand alone amp. I have used it successfully for audiences up to around 100 people, both inside and outside.. It is also what I threaten to use in jams if people don't use appropriately sensitive volume.
My DB has a piezo system and a mounted mic; for bluegrass and other more tone-sensitive genre my first choice is the mounted mic, for louder genre or if I really want to annoy in loud bluegrass jams, I'll use the piezo system. I also use wireless dongles with my DB and PA/amp situations -- playing a DB plugged in and without cables is a remarkably wonderful thing.
