Dear wrm0325: I don't know what phono stage/linepreamp you own, I know that your TT is a good Denon/Cotter.
Now, I owned almost all FR tonearm models including the 66 and years ago ( along other japanese tonearms as SAEC and Micro Seiki/Audiocraft. ) I left goes for very good reasons.
In the old times the audio industry and the inside market in japan was focused on those all heavy metal and long tonearms. Was the fashion and over the time gone out japan to the world market with more or less success. Stereophile/TAS reviewers of those times reaslly did not " care " about and their preferences gone for other tonearms but we more or less " rockies " in audio bought those japanese tonearms mentioned and stay with till some of us learned about.
The 66 is a non-damped tonearm and builded from metal that as a metal is extremely resonant and instead to damps the cartridge/LP/TT self vibrations/resonances/distortions only magnify it.
Yes, almost all those japanese tonearm designs are very good " looking " but that's all especially in the FR's.
Read what an expert posted about, a music expert not only an audiophile:
https://forum.audiogon.com/discussions/tonearms-longer-than-12-inches/post?postid=1289705#1289705what am I trying to say here?: that every one that is listening through the 66, IMHO, is listening a lot higher distortions and less music information from the LP grooves.
I'm not questioning to any FR tonearm owner that I respect all of them and their privilege is that they like those higher distortions. My reference is live music and that's my home audio system listening quest day by day.
For me, the name of the game in our hobby is to lower ( put at minimum. ) all kind of distortions at each link in the audio chain and any one can do it if has the knowledge and skills to discriminate between real music information and those distortions.
IMHO, the standards of home audio/music listening quality level is way different to the standards of those very old times. At least my today audio standards quality level.
Now and with out knowing your whole audio system and music/sound priorities if I was you I put on sale at once the 66 and stay ( for now. ) with the Jelco and put on sale too that Koetsu and that Denon cartridges.
For what you get for that sale I put part of that money ( before to decide for a new tonearm/phono stage. ) for a way better phono cartridge and is where I would like to start.
Only an opinion but the " ball " is in your " field " and you are the best judge.
Regards and enjoy the music,
R.