If at all possible I would recommend using the low level inputs on a subwoofer amp. (AVA's digital preamp has both RCA and XLR outputs so this would be easy to do). If you insist on using the high level inputs, I'd connect them as you have described. If you are using the M225 chassis as the ground (black) connection to the subwoofer amp be sure you have the ground lift switch on.
I looked at the only schematics I have of a subwoofer amp (a Dayton from Parts Express) and found that the negative high level connection input is tied to ground thru a 100 ohm resistor. While the M225 could probably drive this without any problem, the resistor is a small wattage and would probably burn up if the M225 were cranked. Fairly easy to guess that in the case of the Dayton you won't damage a bridged amp by connecting both outputs, but it is not intended to work that way.
Most subwoofer amps have a polarity switch so there would be no need to connect with opposite polarity, but if you have to connect with reverse polarity just connect the M225 black output (-) to the subwoofer red input, keeping the M225 chassis ground connected to the subwoofer black (-) input.
The M225's do not invert polarity but your system / speakers might have enough phase shift that a subwoofer amp sounds better (louder) when it is connected "out of phase". Most do in my experience.
Finally, your mention of image bothers me. If the image is great, wonderful, better than the old amp in most every way then that agrees with most of the reviews the M225 has gotten. If you feel the image is lacking you should be sure to try inverting one of the speaker connections just in case one of the M225's accidentally got wired out of phase (they are hand built).
Dan