It can be tough to make decisions based on demos in-store, not only because it'll sound different at home but also because stores often don't provide very good demos. Frequently things are set up incorrectly/improperly, speaker positioning is usually terrible, the tone controls often used excessively, etc. And many stores will make sure the setup favores the models they want to sell (not necessarily the most expensive, but with the best profit margins).
So many questions here are "what should I buy" types, but it's awfully hard to really answer that. That doesn't mean you shouldn't ask, just that I can tell you what I'd buy but you're the one that has to listen to it. Specifying a price range also would help us advise you.
To be brutally honest, there are differences in the brands & models out there that could matter to a discerning listener, but almost everyone makes a decent model that would serve the average listener very well. This is a great time to be getting into HT, as most companies make superb units in the $400-$500 range. And in the $1k range, the recievers begin to compete strongly with separates.
If you believe MosFets have a particular sound, company literature can tell you which models use them in the output stage. Or, look for a model with preamp outs and add whatever type of amps you like.
Certainly the biggest upgrade will come from replacing the HTIB sub & surrounds. If you have Klipsch mains & center, then by all means by Klipsch surrounds. You might look into a Klipsch sub, but I think for the money you'd be better of looking elsewhere. Depending on your budget, Adire, HSU & SVS make very poten subs, and the new Outlaw Audio sub (a collaberation with Dr. Hsu) is a good sub, too.
