Woops - I made a mistake earlier... must have looked at the wrong data for the subs I was using before. They are 3.0 DCR and not 3.5 DCR as I was assuming before.
With those D4 coils all in parallel I had 0.375 DCR per amp, which is more like a 0.5 ohm nominal load.
With the D1 Nightshades I built I have an actual 0.50 DCR load per amp, which is more like 0.70 ohms nominal.
Even still... it is doing 154.3 dB with a DCR much higher.
I may have to wire the amps down lower and try it that way with these coils. Or use a different amp configuration like four 1500Ds wired down to 0.25 DCR each to get a better comparison.