Had a chance to ask Steve about the impedance rating today and indeed the higher impedance is due to the added damping of the horn base.

6 ohms being a nominal rating, Steve said that the impedance dips to about 3.8 ohms at the lowest.

It's well known here that the decware line of amps are designed to handle low impedance speaker loads. I've heard elsewhere (amongst the DHT crowds) that tube amps in general do not like speaker loads below 8 ohms and even prefer 16 ohms. This is mainly because tubes are voltage devices whereas solid state is more useful in terms of current. I understand that lower impedance speakers didn't become a thing until solid state had a firm hold and it was known that you could increase the power by dropping the load.

The horn base of the monoliths will couple the monitors to the room more effectively and therefore the amp is seeing more load. Not all tube amps come with a 4 ohm tap so i'm glad to see that this speaker combination presents a more universally compatible load for tube amps.