High current at near 0 volts is a lot easier than high current at higher voltages :) This might be why some of our customers had success with the Sovereigns, and we make OTL amplifiers.Not if there's a -50'-60' degree negative phase shift also at 3ohms, then the amp could be looking at nearer 1ohm.
Not exactly how this works, but yes, large phase shifts mean high currents near zero volts. Hard for most amps. Would be nice if we had impedance graphs for the Duntechs.
Bruno Putzey is one of the top class D designers in the world and the Hypex modules are extremely competent. I highly doubt that they could not drive the Sovereigns. Now the modules need power, and if someone put them in a box that did not give the modules the power they need, that's another thing altogether! I'd be a bit nervous blaming the performance of the amplifier on its modules until I knew more about how well its power supplies worked!