There really is no good way to connect things properly with your current setup I think. The X-over you have is intended for 2-way stereo x-over and has an added subwoofer output (mono). The thing is that your Mackies already have a x-over in them.
What you want to achieve is that the incoming signal is split in two parts, one the bit below 75-125Hz (which goes to your subs) and then the remainder of the signal (so minus the bit the subs handle) that goes to your tops (the Mackies). Inside the Mackies the built-in x-over then splits things in two again. Depending on the amp configuration in your active speakers this is either before the amps (if your speakers have two amps, one for high, one for low-mid) or after the amp if your speaker has only one amp. The Thumps are bi-amped so they have an active x-over.
Frankly, while there are some ways to physically hook things up, it’s never gonna be the proper way. Personally I’d try to sell the x-over/amp/subs and replace with one active sub suitable/matching the Thumps (Like the 18S). The sub will have x-over and you can just run stereo balanced leads from your controller to your sub and from there with balanced leads to each top. Done.
Looking at your current gear (Peavey is ok, X-over I know and not too fond of, I have a seriously dislike of the E1800X), I dare venture stating that you will have a well-balanced setup with only one active Thump 18S combined with your 15s that will outperform your current gear. And if you still feel you need extra sub, you can always add another sub at a later date.
You won’t have to worry about how to hook things up, 4 XLR cables and you are all ready to roll.
Hope that helps some.
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