I have tried REL strata 3 with Spendor S3/5 and it worked very well. A friend runs with spendor's own subwoofer (perhaps another consideration). The REL was set to cross at 70Hz, and it was pretty seamless.
The only downside to the AB1 is that I'm not sure how transferrable the sub would be if you were to change the monitors, whereas the REL will blend with almost any speaker out there.
The only downside to the AB1 is that I'm not sure how transferrable the sub would be if you were to change the monitors, whereas the REL will blend with almost any speaker out there.