This is what I think that I want. A 20 to 30 watt, fairly compact amp that will give me clean low bass sound. The purpose of This amp is so that I’ll be heard! I do not need distortion, chorus, delay or any of the fancy things like that. I play bluegrass and traditional fiddle tunes, so I am looking for CLEAN acoustic music. My two gun Arsenal is a Dean four string and Michael Kelly five string. I run the basses through a foot switch preamp/DI box and a 40 watt combo amp. That doesn’t have any depth at all. Yeah, I know that an upright is the right instrument for this application, but I am NOT going to spend my last few years on earth struggling to get Moby Dick through a doorway. No, those electric uprights sound like electric basses. If I’m going to sound like an electric bass I’m going to PLAY an electric bass! It seems as though that there is just NOT a specific solution to what I want, so please tell me how close can I come, and how?
Here are a few of my questions:
Are larger diameter speakers a big deal when considering an amp. Do basses really need humongous speakers?
Is it worth my while to look for an amp with more power, say 40 or 50 watts, in order to have the larger speakers?
Should I consider just abandoning the on board electronics and just mic the bass? Would running the miced bass through an equalizer get me closer to what I want?
Would running the five string, using the onboard electronics but through an equalizer be better? The Dean has virtually no on board electronics, so it’ a moot point. I don’t worry too much about the Dean it’s only my emergency back up device (that I have never used).
Hi @Jim_Comeaux : first, there are a number of threads already discussing various aspects of amps, cabs, combos as well as speaker size etc. By searching for them, you might get a wealth of extra information.
My immediate take is that I don’t think 30 or 40 W will do you any good in order to be “heard”. I assume you want to be heard in - at minimum - a rehearsal situation with other instruments, or even in (not too large) venues when playing live!?!
30-40 W might be OK for playing at home, but that’s almost about it. As soon as you need to “compete” with a drummer, then you certainly want to have more (I think there is indeed a thread about needing an amp to compete with a loud drummer). What made you think that 20 W might be OK for your purposes?
The other thing that immediately comes to mind is that you are (slightly) convoluting a discussion about volume and a discussion about tone. The type of tone you are after might be achieved in several ways, including rolling the tone off on passive basses, EQ’ing on active basses, potentially some EQ’ing on your pre-amp pedal, but as much also by choosing the right strings, choosing the right pickups (e.g., neck pickup if you have a choice) and where you pluck (i.e., closer to the neck than to the bridge).
Just a few thoughts to get this discussion going 
A 20-30 watt amp ain’t gonna cut it. All it takes is one banjo and your 20-30 watt amp will be lost.
As @joergkutter said
When playing bluegrass drummers are not really the issue but banjos will be. 
I play bluegrass with a Fender Rumble 100 and have no issues. Just my $.02. In the end the choice is yours 
Acoustic guitar amps usually works very well. Most acoustic guitar amps also features mic input which usually means it can handle low end as vocal range can be crazy wide.
The 3 models I tried are pretty awesome I own one.
Genzler acoustic array pro
Fishman
Fender Acoustasonic
I use my genzler with my Upright electric it sounds great. I also have the Genz Benz Shenandoah amp that’s quite awesome when I plug in my basses. They are absolutely awesome with Fretless.
I know @Gio has used an acoustic bass amplified for live gigs.
Also @MikeC has a lot of experience with acoustic instruments.
I’ve played a ton of bluegrass and most of it was on upright.
My first question is this:
What is the rest of the group that you’re playing with? What/who do you need to be louder than?
If you’re in a song circle with acoustic instruments (even banjos) I think a 40 watt, deep solid state amp would be just fine.
If there’s a drummer, it will not be.
If any of the other instruments or singers are plugging in to amps or using microphones, it will not be.
Normally I’d agree with @joergkutter and @Celticstar , but if you’re in people’s living rooms just singing with instruments, that 40 watt amp might be enough.
My practice amp was always more than enough for song circles.
No special type of amp.
The Fender Rumble would be ideal here.
No special amp, but something decent. Don’t use a cheap 10W practice amp.