When you say corner loading, are you saying in the corners, or stack them? I can't put them outside my 80's, since my corner bass traps are in the way. I've got them now just inside each 80, with the drivers the same distance from the front wall. It sure looks like a roller coaster ride.
What is strange is if I put all the subs out of phase, 180, the graph is better, and the 40hz hump is not as bad. I've got the 600 back along the back wall again, next to the walk out door. Maybe I'll try the side wall again to see what happens.
I'm just getting tired of moving these subs around.
Look at this way. Moving subs saves you a trip to the gym.
Anyway, If it looks better at 180 phase then I'd leave it there. Also you can get more output from the 350s from room gain down low by moving them into the corners in front of the bass traps. It may make the peaks higher somewhere but its worth a try. Given your volume problems I would suggest installing the 600 into the SMS so you can have more control over the input signal gain.
Regarding the hum have you tried XLR cables between the subs or splitting the 600 from the 350s at the Denon to troubleshoot where its originating? Hum is very irritating and it may be worthwhile to attack that problem first.
