Tom,
I have a Yamaha V-1800 which should be similar to yours. Everyone is right about compression. The Yamaha's default is MAX for dynamic range compression. You want to go into manual setup, then into the sound menu and set dynamic range compression to either Min/Auto or Std (there is no "off"). Mine is set to Min/Auto and I noticed an improvement in dialog over the Max setting. Try it.
I noticed that your 5C sits below your TV; that's ok but you might also want to play with the Dialog Lift feature that you have with your Yamaha. It'll raise (lift) the dialog from its initial location but you can't lower it, i.e., if your 5C was on top of your TV it wouldn't do anything. My 5C is above my TV and I had to shim the back of it so it's pointing down a bit. If the dynamic lift in the AVR doesn't help you could do the same but shim it up a bit.
I also have the 5C at +4 over the 5T; a personal preference for sure. Play with it and you'll find something you like; I'm 60 and I need a bit more ooommph on the dialog Remember, with your Yamaha you can save to memory a number of settings so if you find that stereo music sounds best with a setting that's different from an action movie, just save those settings and you can switch with 1 key rather than bringing up the setup menu and setting everything over again.
I use the Yamaha Sound Field Programs (Entertainment - Movie, Sports, etc) on occassion. I tend to leave my AVR on Straight but there are times that one of the sound field programs sounds better; I find this to be especially true for stereo music but I have also noticed that if I'm watching something like a football game the dialog is better if I switch to Entertainment-Sports. LIke anything, only your ears can decide.
I do find that some television programs (mostly HD broadcasts) have terrible dialog. I've spoken to Cox Cable about it and they're clueless "we just give you what we get". That's not necessarily true because as they add more HD channels they've compressed the content because of the bandwidth limitation. I asked the Cox rep about that and he replied, "huh?". But, 90% of the time everything is fine. I don't have a BluRay DVD. I use a Sony SD connected via HDMI to the Yamaha and have yet to come across a DVD that didn't sound terrific. On BluRay it's all on how well they re-mastered the contect so, like others said, the quality will vary.
The Yamaha auto-setup is semi-useful. From what I've read in here it's not quite as bad as the Onkyo setup but it's still not something you want to rely on. When I first set my system up I used the Yamaha auto-setup and it didn't sound too bad. It got some of the basics down with no problem (small/large, speaker distances, etc) but after it finished I went into the manual setup and used a meter to tweak things a bit.
Overall I'm very satisfied with the Yamaha and the Aperions (I have 5T's in front, a 5C, 5B's and a 10D sub). I think once you get things dialed in you'll be very happy. Feel free to post questions here (there are a few other Yamaha owners) or to contact the Aperion Gurus directly; someone will be able to help.
Joe
If you can't have fun doing it, it's probably not worth doing.