Well you really got bad advice from that twerp!
First off, there is no quality difference between for your short run between a balanced and unbalanced connection, none at all. So that is your simplest solution.
In fact the 10 db attenuator would be correct, if you insist on using a balanced connection. I have years of experience with this sort of thing. When you get an amp like Emotiva that has standard unbalanced line gain on the balanced input, the 10 db attenuator works perfectly. That is why the Shure attenuator was 10 db. In years passed I used these for just these kind of problems. You can tell that is correct, as you say Audyssey is setting your speaker levels at -12 db, which is the max cut. So if the cut is correct then with the attenuator Audyssey will set your speakers at -2db. If it actually needs greater then the 12db cut it now shows, then your speakers might get set at - 3 or 4 db. So if your insist on using balanced connections, those attenuators will do the job for you.