A single M5 hits a valley of 6 Ohms at the low frequencies where I witness the 1500 and 1000 shutting down. Two in parallel go down to 3 Ohms at those low frequencies and 2 Ohms in the lower mid-range. That's well below the ADA 4 Ohm spec.
The saving grace for the ADA is that at low frequencies, the M5 load is almost purely resistive. That's not the case in the lower mid-range but there's less energy in that band compared to the lows so no shut-downs there.
Agreed that shunting boosts the current to a point. Then the voltage collapses and that's the end of that.
The ADAs are well-designed from what I can see.
