There is no one proper way, but I would just use splitters out of the preamp to feed the subs. Left and Right channel.
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?