Is this how a Subwoofer Crossover is supposed to work?

Erik is wrong. He doesn't know what rolloff the sub naturally has. It should be quite flat easily up to 200 Hz if not much higher. 

The subwoofer may be performing as designed. Either way you're not happy so you'll need to replace the plate amp. Minidsp won't fix it. 

Why on earth do you need subs with a pair of MOABS ?

It is NOT setting the roll off.. It is setting the POINT of the roll off.
@ 12 db and 50hz, there is STILL a lot of BASS (ability) in that room.

Set all tone controls FLAT, if you have them. (Preamp)

Turn one sub off. Turn ALL the knobs CCW. Use only (SUB) one at a time.

Split the XLR or the RCAs. Don’t mix the two for now..

If you do, swap the sub XLRs for the RCAs on the mains, that alone could be a problem.. Higher gain...for the subs less gain for the mains.

REVERSE it.. or just use one or the other XLR or RCAs, for now..

0 phase
50 hz XO
0 gain..

The roll off is gonna be 12-24 db, probably 12. 12 db per Oct..

Turn it on AND...

Add in the GAIN only to about the 11 or 12:00 position..

No phase shift...YET... No higher than 50hz on the cross over, YET.

Flip on the second Sub unit. Same thing. ALL 0, and 50 hz on the XO.
Now turn up the gain to the 10-11:00 position.. Smile....

Now better location, placement and very tiny adjustments... The further apart the speakers are, the more likely they will need a "Phase" adjustment, BECAUSE of the Moabs...

AND or

Limit the Moabs to START at 40-60 hz. If you can do that, I know I would be happy. Then and ONLY then would I measure for placement, and placement ONLY... that will smooth the hot spots.
A quick way to limit the Moabs is an inline (cuts below 40-60 hz) simple pro audio way of doing a sound check adjustment. (tip from my little brother) I don’t use inline filters... Option, though..

Setting up corner to corner is a great option too.. A lot less room treatment is needed... Like ZERO....

Regards

erik_squires I can remeasure with one sub and set the crossover to its 200 Hz high point but I’m not sure the results will show much difference but you can never tell.

Depending on the delay and the frequency, you can cause destructive interference. For sure, if you can’t integrate 1 sub, you can’t do 2.

Measuring with one sub with the crossover wide open tells you what your sub is like alone. It’s an important baseline. For instance, if the FR looks like you posted, that’s pretty bad, and you are definitely going to have to do a lot of EQ. On the other hand, if it smooths out, and looks nice and flat-ish to 150Hz or so, you know you have a good starting point.

Integrate your closest sub, so you know what is possible, then blend in your second so your result is as good, if not better than your first sub.

If your overall FR with 1 sub still looks that bad, then you have other issues, and will need serious EQ. Putting in both subs and attempting to hammer the EQ into place though is madness. :)

Trust me. :)