I have the Moon 700i, you find them often at around 6k USD. Best amp I ever had, including pre+power combo from Allnic, Air Tight and Leben I used to have in the past.
I don’t know the 340i, but the 600i and 700i are among the very best there is in terms of imaging and soundstage, excluding uber-high end gear. I had the Gryphon Diablo 300 and the Vitus RI-101 at my place on loan. The Vitus is clearly sounding a notch below the Gryphon and Moon. The Gryphon is in the same league and has a lot of strengths, but for the classical music I listen to mainly, the Moon’s lifelike transparency and delicacy just made the choice for me. But the Gryphon is a terrific amp, have a listen to it.
Also, the reason why you can see the 600i v2 in the aftermarket is because Simaudio offers a full-money back policy if you upgrade within the first 12 months after purchase and 75% within 2 years. The previous owner of my amp went up the ladder to the pre+power solution from Moon. If you’re looking for their top separates, you will see much less second hand gear, because people stick to it.
my two cents