Single vs. Dual Subs

It depends on the room, the mains and most of all, fc and slope.

If fc and slope reach into the lower mid-bass, one sub is localizable and must be placed between the mains.

IMO, 2 subs are mandatory for 2 channel. 3+ is a crowd.

Pray, do tell us all.
Integrated model and connections, Main speakers, room dimensions, DSP parameters, sub models & location

Read Floyd Toole's book before embarking on room treatment.
Are you driving the mains full range?
What type of XO [BW,LR, BE & order] are you using?

• 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.

All IMO:

• the most bang for the buck are the Martin-Logan x series. Complete control set, available Perfect Bass Kit make integration a doddle.
• JL Audio are very good, but pricier.
  
• REL ain't what they used to be and are more jewelry than engineering. Their web malarkey is geared to the uninformed and WAF wimps.
• Never heard or used SVS

My perspective is from fidelity and integration standpoints. If you want to be punched in the stomach [which does not happen when recording a kick drum] I'm outta my depth.

see  ieLogical SubterraneanHomesickBlues for more info and a primer on integrating small subs.

Amen!