Have you thought at all about a SpeakerPower rack amp for your setup? As I see it...
1. They're relatively efficient, even for a Class D (90+%), so you might net a little more power given your wall limitations.
2. Each channel is stable for 2 ohm operation, so you could wire up Ch 1 as your 21" DO setup, and Ch 2 for your LMS and UXL drivers accordingly. Since you'd only be running interconnects to a single amp, this might improve available voltage on the preamp side to avoid clipping that output.