I see some logic there. Actually, the more I've researched the fewer objections I find to using different brands, as long as they are of similar quality.
In other words, don't use a Sony or Yamaha $250 sub to complement an SVS, Velodyne, M&K, etc.
Others says a nice big sub in a prime location is as good if not better than two smaller ones....there seems to be a lot of variablity, and most of that seems due to the specific acoustical environment.
I think only a professional installer would have a sense of what would work best in a particular setup, not that the rest of us could not work that out, through some experimentation, as curegeorg has to some extent. The HK white papers are a bit heavy handed for the average user, but the info is very 'right on' regardless.
I find it an interesting subject, I am not trying to challenge anyone here, at least I hope it did not come off that way. Just questioning the reasoning as to why some do things they do, and learn something in the process.

In my opinion, I don't think adding the LFM-1 would be that much of a problem. But I think you need to assess if you really need another sub. first.
You will see under another topic others have added the sub to their system with no problems. Besides, you have a 30 day trial to find out if will work for you.