I don't like at all how my central channel speaker sounds with my 6700H and I miss the sound I was getting from my Onkyo NR 787. Is is TRUE that as both the 6700H and the SR8015 uses the same Audyssey calibration tool and share some internal parts, that both equipments sound much alike?
I'm looking for the possibility to change my 6700H and buy a SR8015...
I don't think changing from one high end receiver from the Denon/Marantz group to another from the same period will change much in the way they process surround material.
There's an overall center level adjustment setting (also reflected in the speakers levels adjustment menu) you could increase.
DTS:X has a separate centre channel level setting in the surround parameter menu called "dialog control".
Then there's the fact that movie reference volumes are really quite high. This means that without compression the dynamic range is such that to be able to properly hear dialogue you really need to turn it up. Probably higher than many actually playback movies at home without a dedicated movie room and with neighbours to consider. Your Onkyo had THX settings, like "THX Cinema" to remedy issues stemming from mixes meant for theatres played back at home.
To do the same on your Denon you can play with the loudness management options (Dynamic compression in surround parameters and Dynamic EQ/ Dynamic Volume under Audyssey settings)
Lastly, in my opinion, Audyssey sounds best when its correction is limited to only correct below ~300Hz (around the rooms Schroeder frequency)
This can only be adjusted using the MultEQ app though, which is buggy and costs 20 bucks...
Before buying, you could of course just try with Audyssey turned off to see if that improves the centre channel.