Single vs. Dual Subs

@m-db, The much better solution is using digital bass management which includes room control and digital crossovers which are far better than analog crossovers. From less to more expensive you have MiniDSP, DEQX, Anthem and Trinnov. Check them out. If you are a person who is stuck on analog only just forget about it, my mistake. 

I'm running a pair of Tekton 4-10 bass towers & am very pleased. Prior to this, I was using a single REL sub. The bass towers improved not only bass (as expected) but also soundstage width, depth, instrument separation, & top end clarity. A very nice improvement top to bottom. I have these in a dedicated room that is around 15' square.  

@millercarbon Lose the attitude. You’re an a**hole.

@ieales
I am driving the mains as full range because I have no choice.
"What type of XO [BW,LR, BE & order] are you using?"
I would answer your question, but I have no idea what you are talking about. I assume XO is a crossover, but what is everything else?

• XO is a crossover.
• BW Butterworth, LR Linkwitz-Riley, BE Bessel which are types of crossovers. Each type has a different phase and group delay which impact how well the subs integrate with the mains.
• Order is dB/octave and controls the slope.

The  Vienna Acoustics Mozart Grand SE claim 30Hz bottom end. It's a ported design. The subs are unported and thus the bottom end characteristics are radically different.

It's a good move to use the miniDSP. Turn the sub's XO frequency all the way up and set Phase to 0.

You should use something other an integrated so you are not driving the main speakers full range. With a 70Hz corner in the miniDSP, I surmise there is a LOT of incoherent bottom and mid-bass in the room. 70Hz is easily localizable.

Having used full range monitors in recording studios and subs at home, it my experience that maximum musical impact occurs with coherent low end.

Good luck.