Sometimes Hard to Drive Speakers are a Gimmick

I think that this is more prevalent in tower speakers with 2 woofers connected in Parallel.  The idea is to get more bass sound/dynamics in the lower frequencies by pulling more current.  I think this configuration is there to get a more "fuller" sound.  With this type of parallel connection, if you have the right amp,  I believe you can get bigger sound with smaller woofers. But, actually, I do not know anything, this is just my assumption. 

@hjdca 

You are absolutely right that dual woofer designs end up wiring the speakers in parallel, so hard to get any better than 4 Ohms, and I kind of talked about that a little. 

Still, plenty of dual woofer designs which have better impedance and don’t have a missing resonance hump. 

My own SNR-1 uses 4 Ohm woofer instead of 8 specifically to get more output and make it a closer match to the tweeter.  That’s totally legit.  Making your crossover harder to drive just to make it harder to drive is not OK. 

interesting observation but in my 40years of high end audio experience totally irrelevant today

WHY?

today high powered amplifiers which can put power into low impedances are everywhere

So most loudspeaker designers simply don't care it is not a consideration

and most high end loudspeakers will be used with the appropriate matching electronics

today a high powered powerful integrated can be purchased for1500

a Nuprime ida9 can put out400W (8 Ohms), 500W (4 Ohms) so that means under 4 ohms the amplifier can still be putting out power 

as long as you have adequate cooling most amplifiers will have no problem and even in the case of a low impedance dip it still depends on the frequency content, amount, and duration, of that signal

take away not the same issue with most modern high quality amplifiers.

Dave and Troy

audio intellect NJ