First item - sincere thanks to RickCathey and Ajax for their responses. The thought & effort that went into thier posts are recognized & appreciated.

My situation is one of transitioning from many years interest in stereo to a new 5 or 6.1 system. Not really into HT (yet), so music is my thing. I'm upgrading one thing at a time. Started by replacing an old Sony receiver with a Marantz 6300, & was really surprised at the immediate difference in sound quality (I expected this to occur as later upgrades happrened). Axiom M3's were the next purchase - more improvement ! My original main speakers are Epicure Model 4's (anybody remember those?), & I'm using them as "surrounds" for the time being.

Latest upgrade - just ordered the new SVS PB10 sub. This will be my first experience with a sub (a sub-virgin? , & I'm really anticipating it ... common reaction for first-timers I suppose .
So that's where I'm coming from re the Feedback Destroyer.
I understand that room responses are the biggest and least controlled factors in our sound systems. I don't have a sound meter yet, so I don't know what my specific situation is, but will be getting one soon. Links you all have provided may answer my next question, but there's a ton of info there & I've only skimmed it so far. But it appears that the Feedback Destroyer addresses only low frequencies.
Is that a hardware limitation, or is it all that's really needed in home systems? Put another way, what about room anomalies at higher frequencies? Isn't there a need for smoothing room response at higher frequencies as well as lower? And can the BFD do that?

- Thanks again for eveyone's help -