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You may be better off, much less strain on your receiver that way.
I had the same problem at one time (5 VMPS speakers plus 1 VMPS sub) and an under powered receiver.
My mains were VMPS Supertowers, the lower section of these are the large VMPS subs...Why would I run them small?
My receiver had pre-out for all channels so I just added a dedicated two channel amp to drive the fronts full range.
I set the center and rears to small and let the sub deal with their deep bass...this left the receiver with only 3 channels of 80hz and above work load....sounded fantastic.
You will need to do one or the other depending on your room and speakers I would say.
Following the advice of the expert who installed my Velodyne HGS-15, I set the pre/pro to shunt everything below 80 Hz to the sub, and it had the effect of really opening up my KEF 104/2s in stereo mode and enlarging the soundstage they generate. Integration with the sub is seamless, and you cannot point to the sub as a source of sound. I highly recommend letting the sub do what it does best, and freeing your other speakers of low frequency duty.
A different angle...
With all the speakers reproducing full range bass, woofers are in many places in the room. Standing waves and cancellations you didn't plan on may become evident. With all the bass coming from one location (the Sub) it's possible more locations in the room would sound better (instead of one sweet spot) Experimentation may bear this out. If all your speakers are indeed capable of reproducing down to 20Hz it could very well be an issue. It's even tricky with multiple subwoofers.
Your speakers are full range speakers with active 12" subs built in, correct? You should be able to get very good results crossing these over lower than 80hz. Because of the effieciency of your bass system here, You should probably try to cross lower. Yes, experiment and see. You may find they're very powerful and dynamic even full range, set to "large". If your receiver selects at different freq's, like 40,50,65hz, etc, you should try those. I would.