Judging from your sig, you're on a converted Road Glide, correct? I have no experience with a RG so I can't say if it will fit or not, but the Power Acoustic Razor series is a great amp and not too expensive. The 2000D4 is just a shade over $100, and the 3000 is better still. I think Gannicus will back me up on this. That said, they are pretty big and it's a tight fit even with a batwing. They're of a similar size as the SoundStream amps but not as pricey.
+1 If you can get either the PA 3000D or the Soundstream 1000D in your faring. Those are the amps you want in there. Plenty of power to drive any speaker out there at a very good price. The amps I'm referring to are the newer versions of the amp John recommended and are an inch longer. If they won't fit go with Johns recommendation and you'll be a happy camper either way!!
Hard to beat the prices of the Soundstream/Power Acoustik amps though.
The 4180 is the best amp on the market IMO, not only because it outperforms anything else, but because it's the last one you'll ever need to buy since they have a lifetime warranty. Not to mention they have stellar customer service. I couldn't afford one though, and went a different way.
I really like the 2180 I have now, so I think I've decided to go with a second 2180, as it's small enough to fit beside the glove box in the fairing, and the BT479 6x9 speakers.
Just need to make the purchase...that's the hard part!