Have been using stacked subs in my HT room for almost a year now. Still sorting the room out, and don't have long enough cables to put them in the proper place.
To sort out L/C/R speakers, the subs had to move, so they go stacked in the corner of the room. Yes, ran DSP after. Both subs have footers under them, to isolate from each other.
They sound just fine, get all the bass I could ever want and then some. Once stacked, both needed the gain to be turned down. Think it's a little cleaner, but these are movie subs, not musical.