On the center level thing, are you raising the master volume or just the center channel level for dialog? Is the center positioned optimally? How far are you from the speakers? Some dialog is simply mixed that way, too.
Ideally the three across the front would be the same speaker....the typical center design is a compromise for horizontal placement under a tv. For three towers an acoustically transparent screen would be needed, tho sometimes you can accommodate three identical bookshelf style speakers with a tv.
I wouldn't worry about getting new speakers and using the old center (or just go phantom center) while you save up/decide on the next step; Klipsch usually has a matching center for their various models.
As to adding an external amp to your avr, you chose the wrong avr as it does not have pre-outs (except for subs) to accommodate such. The Klipsch tend to be of sufficient sensitivity where your avr is likely quite sufficient, tho.