A couple of things come to mind that could be affecting the performance of your speakers:

1) Room acoustics/interaction
2) Front end
3) Calibration

As mentioned by GM and Pat, there's a very good chance your room is simply interacting with your speakers in a manner that smooths their perceived response. Combine this with a front end that may not have the same power characteristics as the dealer's demo system and you end up with limited dynamics, especially during transients. The Kappa's were pretty well regarded, the center in particular, so I really don't think it's the speakers. Still, I've always found DefTech's to excel in HT as they are voiced pretty aggresively. To be honest, I'd probably be happier with what you've got than the Mythos, but to each their own.