I would just get a 600Wrms mono sub amp that is 2ohm stable. I have a MB Quart PWE 354 15" sub that can handle 800wrms (400wrms per coil), and I am powering it with a JL Audo 500/1 amp that puts out 500wrms. The sub is under powered, but it does sound really good. I can hit up to around 132dB. That's not as loud as some of my previous systems, but that's more than enough for daily listening, and my goal was more sound quality with this build, rather than just SPL.
That Crunch amp would be very much under powering your MB Quart. If you wired each coil to a separate channel, you would only have 87.5wrms per coil. That amp can't handle 2ohm loads bridged, so you would not benefit from mono connection. I would look at any amp that can handle 2ohm mono or has enough power (at least 200wrms per channel) for each voice coil.
I've had good luck with my JL Audio in this car. I've also really liked using Sound Stream amps in the past. I had the Sound Stream Reference 500. They are build to handle just about any speaker configuration you could throw at them. Mine could handle down to 1ohm mono loads. There are many other amps that can provide the power you need and handle the stereo or mono connections.
I've got an old Earthquake PH-D2 2000wrms mono amp for sale. I had it connected to a Cerwin Vega Stroker 18 several years ago. Now the Stroker 18 is being used as my home sub.
Good luck with finding an amp for your system.