Thanks guys. I wasn't even going off of specs actually. My thing was that I sometimes notice that the highs are flat sometimes. The mythos and 8080's are rated at the same 30khz but I figured because of the solid cabinet it would help out.
Like I said previously, you already have pretty good speakers. The Mythos is a lot of money for no upgrade or very small upgrade. For the price of the Mythos, you could certainly find speakers from another brand that will out-perform either the Mythos or the STs.
Sometimes, you spend a lot of $ to find these things out for yourself. That's just the nature of the hobby.
Again, if you want to improve your current setup, go with a big-boy sub or 2. Yes, I know the STs have the built in subs and amps, but it still won't compete with a dedicated sub from Rythmik, HSU, SVS, etc. If it were 100% music, then no sub needed. For movies, sub is needed.
As far as the highs being flat, have you run the RC on your Marantz? If not, then try that. If you have, then try to disable the RC and see if that corrects the problem on the highs.