Amp or DAC?

No never had the Benchmark amp. Not sure what you mean by control of the woofers, what speakers.

I tested them against two bookshelf’s - LS50 and Unifi UB5, since these are supposed to be somewhat ‘hard to drive’

I just meant bass response 

cd45123, I had class D amp and currently have AHB2.  Both have excellent bass control but AHB2 has a little bit better extension.  I cannot explain it, since on paper it is only few Hz difference (5Hz vs 0.1Hz), but it is audible (same source, speakers, room).

From the point of membrane damping output impedance of the amp has very little effect on it and anything above DF=15 should be fine. 

AHB2 - is a wonderful amp - dynamic and clean, but don't expect "warm" sound.  It was designed to be accurate.

Excellent bass response down to about 35hz slight roll off to 24hz then it drops off. My active speakers use Pascal Class D amps. The Benchmark is very hard to beat in clean accurate amplification if I ever go back to passive speakers it's the first one I'd look at but I don't see me ever going back but never say never. 

I went from a 225 watt anthem to a 75 watt Belles and the belles sounds better.

I went from a Classe CA-150 to Classe CA-200 and the 200 was better.

I went from an Adcom 545 to 535 and the 535 sounded better.

No way to tell.  If all other things are equal, more watts is most of the time better.  I’d rather have 100 clean watts with great current than 1000 watts with no current.

Amps and Dacs are different so like Doug said, no shortcuts.  I.e. $50k speakers connected to a $200 receiver isn’t going to sound as good as a $5k pair of speakers connected to a $5k integrated.

Put another way, michelin pilot cups on a toyota camry isn’t going to make it handle like a porsche