The C9 is 2.5x the price unless you find a used one. It also provides a richer, more vibrant sound; but with a warmer signature as well. It is quite powerful, too along with more connectivity options. Battery power is very good on both, but the C9 allows the user to change rechargeable batteries on their own, giving longer battery use than the OEM batteries.
If I had to choose one, without $ being an objective, the C9 wins. I do love that sound. But if price comes into play, it’s the Ampersand hands down. Or, you get the Ampersand and the M8V2 and still save $500 over the C9. Then you could get a really decent DAP or mid-fi headphone/IEM like the HiFiMan Edition XS or a used ZMF or Audeze. For the same price spent, I’d go with the CEntrance products.
Cheers and I hope this helps.