The RX-A6A has the PEQ feature that supposedly let you use a total of 6 (if I remember right,
@AcuDefTechGuy can correct me if I am wrong) bands on top of YPAO's auto RC/EQ.
So if you work on PEQ, and better still, use REW to do it for you, then input it to the unit, you should be able to do very well, at least in the critical range of 20-200 Hz. It is actually fun to play with using manual PEQ with the auto EQ, people do that with Audyssey too.
By the way, the Cinema 50 seems to be a very nice unit, if you can live with a few features missing in the Cinema 40. As to the power output, as
@TLS Guy alluded to, the difference between the 2 is negligible, wouldn't make any difference in probably 95% of use cases. If you do need more juice, just grab whatever cheap power amps to the channel you need and you are done.
It seems to me your dealer is quite fair about this, because while you are right the SR7015's equivalent is the Cinema 40, but that's only in terms of the convenience features such as the 2nd larger display, and the power amp power supply section (the power amp modules are actually the same), but the Cinema 50 offers 4 subwoofer output channels vs the SR7015's 2, and Dirac Live pay upgradable, that, in my opinion, more than offset the slightly more powerful power supplies and a few nicety features the SR7015 has.
You obviously prefer the Marantz brand over Denon's, so I think the C50 would be your best choice and you will be very happy with it, again, if you can leave with those few missing features, some people just don't like the smaller display.