Use your Main balance output to your MC 501s, use output 1 & 2 for your subs. Make sure to use the R channel of output 1 & 2 interconnects for your right sub and do the same for the left, I use this configuration with my 2 JL Audio E112 subs using a Mac C50 preamp sounds fantastic.
Dual SVS Subs with Mcintosh C2500
What's the best way to connect C2500 preamp to dual SVS SB2000's subs?. There's 3 sets of balance outs. Balance out #1 & #2 goes to two MC501’s. The top unused main out (balance); should I use XLR to RCA splitters to the subs? or use RCA cables, main output #1 and #2, directly to the subs? What's the proper way?