I did a little looking, and I found you what I would consider to be a sweet setup that is certainly in your budget:
Pioneer DEH-P7600MP - it's not the best looking head, IMO, but it's packed with great features. It has a built in amplifier so you can run your factory rear speakers. It also has a built in crossover, so you can keep the bass out of those speakers and keep them from distorting. It also supports MP3/WMA playback. It has a sub level controller too so that you can turn it up or down depending on your tastes.
Price - $250
http://www.sounddomain.com/sku/PIODEHP7600MP
Phoenix Gold Tantrum 400.4 - Sounddomain is blowing these amplifiers out right now. This line has been discontinued by PG, but it's an awesome amplifier for a great price. This is not a flea market amp. Does 50 x 2 to your front speakers and 200 x 1 to your subwoofer. For only $40 more you can upgrade to the 600.4 - does 75x4 or 75x2 + 300x1.
Price - $200
http://www.sounddomain.com/sku/PHOT4004
Image Dynamics ID10v3 - It's no secret that I'm an Image Dynamics fan. They make one of the best sound quality woofers out there right now IMO. The ID series is very affordable and sounds great. They are also known for using very small enclosures.
Price - $100
http://www.sounddomain.com/sku/IDSID10V3D2
Q-Logic Enclosure - You gotta put that sub in a box, and this Q-logic is just the right size. It's very compact as well. You could always build your own, but if you don't want to, here's a good alternative. They make one of the more solid pre-fab boxes around.
Price - $55
http://www.sounddomain.com/sku/QLCQLH6510SE
Knukonceptz Wiring kit - At minimum, you will need a 8ga wiring kit to run the t400.4 (I'd recommend a 4ga for the t600.4). Knuknoceptz offers nice wiring kits at a great price. They are very reliable and reputable. This kit comes with everything you will need to get things started.
Price - $24
http://www.knukonceptz.com/productDe...prodID=KOLAK84
Now, all you have to do is add your front components. Sounds to me like you have done some listening and you like the Bostons.
If you take the list of products I've shown, you have a grand total of $630 + shipping. That should leave you plenty of budget to make some changes and to add in those front speakers. Another thing to keep in mind - everything I have listed is authorized and comes with a warranty. No refurbs, no b-stock.
If you still think a subwoofer isn't for you, then you could easily swap the ID10 and the box for your 6x9's.
I hope that helps you out. Again, good luck with your system.