The ART-9 is a high-end cartridge and it's possible that it's not a great match for the Vincent PHO-8. I see the nominal MC loading for the PHO-8 is 100 ohm and the nominal impedance for the ART-9 is 12 ohms. The "10x rule" would suggest that the phono loading should be at least a 120 ohms and phonos are known to vary from their stated specs a bit.
Besides matching the turntable tonearm, make sure that the phono is a good match to the cartridge. I don't see any red flags in using the high-output Opus3 with the Vincent PHO-8 and several members have used the Opus3 to good effect with the StudioDeck tonearm.
Yeah, cartridge matching is one of the more fiddly parts of vinyl enjoyment.