I agree with above poster...
A home theater in a box is typically for someone who just wants to plug in the nice, color matched wires, and be done with it...
But you're already on an upgrade path.... Good for you, you have a decerning ear!) BUT trying to keep parts of that HTIB system just doesn't make sense to me... If you don't like the sound, I would think changing out the amp will make almost no difference - except maybe to emphasis the lower quality of the speaker?
I would say, - just sell the HTIB to someone who wants that type of system and use the money wisely to start a good system for yourself.
Will you miss the Blue Ray? - maybe, but I'd rather live with a $40 upconverting DVD player (what I happily use now in a ~ $5K 2.1 system) than cobble in a HTIB unit...
I think you mentioned 7.1 too and at the same time expressed concern for budget ? Well, my opinion is again to take baby steps - get the amp you're already planning on, and add a pair of fronts and perhaps a sub. Then later add a center and the back surrounds...
Don't worry - with today's sound processing - you'll be much happier with two good fronts than you would be with a whole circle of too-cheap speakers...
Buy the best you can and cry once.. and buy it in smaller chunks. Otherwise, I think you're going to be throwing good money after bad.
say bye, bye to the sony, please!