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.