Most simply, I do not consider the Behringers to be high end equipment. I do consider the Merlin monitors high end equipment - although they're typical in that there are certainly lots of other good monitors. I'll stick with my opinion that adding a Behringer to a system with Merlins is not good system matching. It's degrading, in fact. Now, with Ref 1s or something, okay.
Rumble filters usually meant getting rid of tonearm resonance to ease low frequency demands on the amp. Of course you are correct that, say, Paul Klipsch's tech papers were talking about doppler distortion as it affects frequencies well above the lows from quite early on. The Cizek literature - remember I specified advertising material - highlighted the use of a sub to clean up the midrange. That's the first advertising material I can recall doing that, but my memory is almost as fried as Paul Klipsch himself. Perhaps you can name an ad talking about this aspect of a sub which I have forgotten from before the Cizek years, but remember that subs themselves (in the context of sats) are a relatively recent phenomenon, and they were and are usually sold as ways to add low end (even though most average subs are just woofers, not subwoofers.) We can argue whether Fried or M&K came first with the sub/sat aspect.
The good news is that there are better crossover options than the Behringers if that's an appropriate course. I never did get an answer as to how the Merlins are being used or why they're having problems from the owner. He did post back saying, you're probably right, a crossover probably would complicate things. I don't know the reasoning behind that decision in terms of his situation with the Merlins, like how big the room is and all the rest. For all I know, he's not having any problems with the Merlins...maybe he's having undiagnosed problems with the amp. Or maybe the room is huge. I just don't know.
The Merlins are fairly precise products.
