He could try to determine the natural rolloff of the 4-10 and use a complementary low pass on the XLS1500 to feed the 18 incher.
That's a great idea
Could you post a link to your speakers?
It will be on the ampeg website....http://www.ampeg.com/
There are quit a few 4-10 speakers on the ampeg web site so if you want us to help you configuring the amp you need to point us to the exact model speaker you have...