What amps do you use to get that great growly "mwah" sound?
In order of importance, IMO, and amp is part of it, for sure:
-Good fretless bass that's set up well.
-Good amp with frequency response/eq that isn't overly bassy.
-Personal ability/technique/touch/style.
-Roundwound strings (maybe optional).
Without all of the first three, it's going to be hard to do, IMO. Not impossible, but harder that it would otherwise be if you've got the first three, for sure. Again, item #4 is perhaps optional, depending on the first three.
Now to the issue of amps, specifically. IMO, brand, wattage, speaker configuration (unless it's like a 1*18, which you don't want) are of little if any importance in getting that sound. What is important is the ability of whatever the amp/cab is to allow that natural midrange to come through. I've gotten that sound through my main rig, my keyboard amp, my Harkte combo amp, and directly through the PA. IMO, setups, whatever they may be, with the least coloration are best suited to a "great growly 'mwah' sound"... ...at least that sound as I understand it to be...