Sounds like you've put together a respectable first system and are starting to get bitten by the audio bug--congratulations and God help you. In other words, welcome to the club.
Although I don't necessarily agree with his alternative speaker recommendations, I do agree with Mdomnick that if you're starting to get more serious about 2-channel performance the biggest improvement per dollar AT THIS POINT will come from upgrading your speakers(not that the Bostons are bad at all, just that there are much better out there--see below). It sounds like you've got a preferred dealer in the area that carries a lot of mid-fi brands and maybe a couple hifi makes(maybe like a Tweeter?), and if this is the case I'd strongly recommend you get out and hear some other brands such as Silverline, Coincident Technology, VonSchweikert, Talon, Gershmann, Thiel, Soliloquy, Audio Physic, Merlin, Triangle, Joseph Audio, etc. Although these may be outside your price range for now it will give you a much better frame of reference for what is possible with a high-end stereo system, and more importantly you'll start to uncover what characteristics are important to YOU and that you would like to have in the speakers(and system) you ultimately choose.
Upgrading your amp and/or preamp now will provide an improvement in your current system, but it will be nowhere near the level of improvement you'll get by hooking better speakers up to your current equipment. As your system evolves other pieces of equipment will become relatively more important(that's why I wrote "AT THIS POINT" in the previous paragraph), but you've got to get the speakers right or you'll constantly be playing the catch-up game(trying to coax certain sound characteristics out of speakers that don't naturally go that way by changing box components and/or cables). Also, there's a good chance any amp you buy now may not be a good match for whatever speakers you fall for in the future, and that would be a big(and expensive) bummer. In short, make sure you get the speakers right and the rest will follow. Doing it the other way around is MUCH more difficult. So my recommendation would be to take some time to hear what's out there and save up your pennies, then go buy the speakers that knock your socks off and build your system around them.
Here's one last thought and it won't cost you a dime. If you haven't done so by all means experiment with speaker placement in your room. Sounds like you've got a TV between your speakers, and if that's the case try pulling the speakers out a foot or more past the front of the TV and see what happens(you don't have to leave them there obviously, but just play around and see what happens--even try a couple feet past your TV if your cables are long enough). You should also experiment with toe-in if you haven't done that either(pointing straight ahead you'll get a wider soundstage, while angling them inward will improve center image fill and focus). In my opinion, next to speakers your room characteristics(shape, size, decoration, etc.) and speaker placement are the most important variables in your system. Hope this helps and best of luck in your quest.