Lots of activity, Ichigo. I'll comment on a few points. First, to clear up the terminology a bit, audio amplifiers and pre-amplifiers have fixed gain, and so they don't have "gain controls". The knob on the sub that makes things louder or softer is connected to a variable resistor which acts as a volume control by varying the amount of incoming voltage(from the receiver pre-out)which is let in to be amplified by the fixed amount of gain. You shouldn't have to set this above about half way, and the -12 adjustment that Audyssey made indicates that next time you run Audyssey you should set it a bit lower so that the end result is closer to 0 than -12.
The difference between the front left and right speakers is rather extreme, so check the right one carefully to see if possibly one of the drivers isn't working.
Generally, except in a square room(one poor devil here, who shall remain nameless, has one)the corner is the best place for the strongest and smoothest response from a single sub, so leave it there.
As for a suggestion of a CD to give your Axioms a good workout and being great listening too, consider
this performance of Holst's The Planets, available at bargain prices.
