Sub best at high crossover?

Impossible to question what your hearing or your taste in fidelity. I'm confused as to what your seeking here.

I owned a DD 18 and now two DD 12plus. I've never tried running them or any other subwoofer system full out. I mistakenly ran the DD 18 at 80Hz which bloated that region from the first moments. A touch of #1 preset put it right back in the groove.

Your speaker manufacture is rather vague regarding the frequency response of your speakers other than to say they have response down to 35Hz +/-? and that they are a ported bass reflex in design. Hardly what I would call full range, but thats me.

If your speakers are down -6dB or more at 35Hz and considering your rooms acoustical nature then supplementing that region would make sense.

Dbphd, that's my experience. Even if a speaker system actually measures reasonable output at 25Hz the room usually requires gain in that region.

I've gone round and round with some folks on Bass forums regarding their electric Bass cabinets which generally begin rolling off at 40-50Hz. While low E is about 41Hz there are fundamental overtones that reach well below and even further from a Double Bass.