If the uppers are begining to roll off at 60hz, yo likely have a ton of overlap between them and the subs.... unless you've got that c/o dial set really low.
You should try crossing them at the -6db point of both the uppers and the subs, or at least close to it. If you cross at that -6db point and the sub woofers and upper speaker woofers are in phase with each other, it shold raise the response in the cross over area back to "flat". Right now, if your mains are beginning to roll off at 60, depending on how steep that roll off its, yo should actually be crossing somewhere below 60.
If you want to cross higher up and have the subs do a bit more work, run a small inline cap on each of the amps input, cap value will vary upon amp inut impedence. This will also take some of the work load off your main amp and your upper section