I would suggest that you demo a few sound bars before you consider buying one. I know you're a 2-channel guy, but that is why I'm pretty sure you will not like what you hear.
The timbre matched thing can be a little confusing. The center channel doesn't carry all of the dialogue, but it averages about 80 percent of it. With multi-channel sound, they can direct sound to the speakers, and the amount of sound as they wish. So if somebody is speaking on the left side of the screen, the mix may be 70 center, 20 left, and 10 right. That's why you try to timbre match.
I would suggest you get an entry level AVR, and don't worry about pre-outs right now. Best Buy open box for 300 or less, an a good manufacturer for instance. I would then use the advents for the front mains, and maybe try a Monitor series for the center. It won't be timbre matched, but I think that would still sound better than a sound bar.
There is a really good buy on the CSI A4, at Polk E-Bay,but again it won't be timbre matched. Then again, I think if you hear, and play with an HT set-up a little, small Monitors or RTI A's may be finding a new home. There really isn't much difference trying to get good/great sound quality from 2-channel or HT. Just depends on how far you wish to go. Sounds like you want a little improvement on the HT front, and it's still sound quality you're working on.