To the op i have a REF Phono 2se > REF 5se w Kuzma CAR-40…. imo running single ended with your REF 6 is leaving a LOT of performance on the table….
Probably no coincidence both my main phono stages use FET front end Tube back end… as @lewm said …. The relatively high output MC i use allow me and some close friends who use both those and the low output Lyra to evaluate the impact of FET…. my own judgement is yes there are artifacts but trying to hear them defines audiophile vs music lover…
best to you in your search