I own 2014 Duo Mezzo's non HD. I own Audiopax 88 Mk2s, Nagra VPAs, and am currently running AudioNote UK Shinris.
I've also demo'd in my system, Luxman KT 88 limited edition (not bad, but kind of thick sounding compared to the Audiopax 88s), Dartzeel integrated (SS) (very good) and the Audio Research GS150 (surprisingly quiet and very good sounding), First Watt J2 (Meh midrange, thin sounding, but pretty good bass and highs)
The Nagra's are 60 watt PP 845s, and very good, with amazing bass. They sound however, very slightly tip up on the Duo's. They were really designed for 87 db Wilsons so are kind of overkill with the 107db Duo's. On the other hand, I much preferred their sound over the 8 wat Shinris on a friends 96 DB ANE speakers.
The Audiopax 88 are dead quiet, very good sounding but don't go as deep as the Nagra's. They were one of the favorite combos with Avantgarde back in the early 2000s and are still a very good match especially at their current used prices.
The Shinri's are a very rare and expensive almost top of the AN UK line 8 watt SE 300B Monoblock from Audio Note UK.
I thought i needed more power based on my experience with the 30 Watt Audiopax amps but was simply stunned when i tried them with the Duo's. All the deep bass I could ever want, very extended and silky highs and the most amazing mid range. I routinely listen to 100 plus DB rock and orchestral music and the 8 watt Shinri's have never run out of gas.
I ended up buying them and will be selling my AudioPax and Nagra VPAs (unless I get a second system with lower efficiency speakers).
So while the conventional wisdom is that 8 watt 300B are compressed bandwidth and syrupy, if you can get your hands on the Shinris they are a better match with the 107 DB Duo's than their own 96 DB Speakers. The Shinris are very expensive but still cheaper than the Viva Auroras, and they only weigh about 35 lbs each compared to 100lb Viva