Despite being published 12 days ago, Ω missed this excellent writeup. Overall, Headfonia agrees with Ω: DUET is a fabulous amp for IEMs. Our opinions divaricate on the point of DUET's performance with headphones. This is Headfonia's take:
The Theorem 720 has a sound signature that more closely resembles the ALO Rx MKIII-B+. For any number of earphones/headphones, that is a good thing. It is muscular and detailed. But so is the DUET. In fact, the DUET sports a lower noise floor and slightly better stereo image when driving high-current loads. That makes it a better amp for both your Audeze headphones and your low-Ω IEMs.
Ω's review is coming soon. While it will ooze sincerity, it will be almost complete ohmage. This is Cypher Labs' first nearly flawless product. 720 may have amazed me with its resolution and stand-up stereo image, but DUET sacrifices very little in delivering better across-the-board performance in addition to better management of circuit noise.
Effusive though the above paragraph may be, I believe it is important to point out that performance-wise, DUET has no Cypher Labs equal. Sound reviews are, by nature, subjective. Actual performance isn't.