Dean, I wasn't aware of your high frequency roll-off problem with the Onkyo. Although it's correct that Onkyo doesn't provide a separate manual setting to specifically choose between the "Audyssey" and "Flat" curves, that doesn't mean that it can't be done to suit your preference. This has been discussed in several posts, especially on AVS, and the best summary of the options that I can recall is at p.366 of the "Audyssey" thread
here , where Rickard from Stockholm describes his excellent study of the issue. You'd presumably be interested in his "a" and "b" categories which allow selecting the "Flat" curve. For example, for music, DPLIIx THX Music could be used.
