Subwoofer active line level input?

I have recently upgraded my HT subwoofer to a servo martinlogan iGrotto, now I have a spare Martinlogan dynamo 300 for my computer 2 channel setup (which is surprisingly musical), to compensate the polk LSI7 in the lower end.

My integrate amp NAD C316BEE does not have LFE, after some research I find out it is possible to use active line level input on my subwoofer. So basically one pair of speaker wire hooked into the LSI7 and the other pair hooked into the subwoofer.

It sounded pretty good.

Here is my question:

The LSI7 is probably getting the full signal, means the bass in being reproduced twice?

The high end companies like REL actually recommends using active line level input instead of LFE, for getting the excat same signal, how do they deal the problem I described above?

Thanks!