We have enough customer running Quads that I can safely say that they are a great combination.
We originally designed the amps to be used optionally with an outboard autoformer. Thus years ago we marketed the autoformer as the Z-Music autoformer.
The design of our amps is based on the idea that feedback adds distortion that is easily heard by the human ear as brightness and additional harshness.
So we designed the amps to have good linearity without feedback. Triode power tubes, single stage of gain, class A2 operation, fully differential from input to output and direct-coupled output.
It was obvious that such an amplifier would have a higher output impedance (we of course built amps with feedback, some of them switchable, so it was easy to hear the detrimental effects it had). So the Z-Music was designed to allow the amps to deal with lower impedances. It was pretty effective; in his review of the MA-1 in The Absolute Sound Steven Stone used the MA-1 with a set of Apogee Full Range loudspeakers, which are 1 ohm impedance- and it worked very nicely! Other than impedance, that speaker was easy to drive.
We don't make the Z-Music anymore as the ZEROs had slightly better performance. Neither product was/is expensive, so this has proven over the decades to be an elegant solution for lower impedance speakers coupled to lower powered OTLs and other amps like SETs that normally have a higher output impedance due to no feedback.
We occasionally get the question- doesn't that eliminate the best qualities of the OTL? And the answer is no because an output transformer like the ZERO or Z-Music isn't possible unless the rest of the amplifier already has a fairly low output impedance and no DC at its output (both devices don't block DC as they only have one winding, and the turns ratio is extremely low so no distributed capacitance to kill the bandwidth). As a result, both products have bandwidth that is well in excess of the amplifier used with them- the ZEROs go for 2Hz to 2MHz! Usually the output transformer is the bandwidth limit of the amp but with this system that isn't the case. In the meantime the amp still has all the qualities that make it work as I described above.