Which tonearm should I keep?

One consideration is effective mas. Low mass arms run around 5-7 gms, intermediate mass arms such as the SME V, Linn Ittok and Ekos, Rega RB-300 and descendants, etc. run around 10-13 gms and are generally what most modern MCs are designed around.

The Fidelity Research FR-64S is probably the least versatile of the tonearms you have listed. IIRC when Hi Fi Choice tested it back in the 1980s it had an effective mass of around 35 gms, which makes it usable only with very low compliance cartridges unless you have a record flattener or a vacuum platter to flatten all your records. It's still relatively popular for such cartridges so you shouldn't have any problems selling it for a good price.

You don't indicate which other version of the FR-64 you have - the FR-64fx, which is made of aluminum rather than the stainless steel of the FR-64s has an effective mass of around 20 gms, which can be reduced using a lighter headshell (the standard headshell has a mass of around 14 gm). The Hadcock is a low mass arm IIRC and is supposed to be an excellent match for the Music Maker, which makes sense because the two are made by the same manufacturer. The Technics can be low to relatively high mass depending on which armtube you use, so it is probably the most versatile in terms of matching for different cartridges.

Hi Rich. I think that you should first deciid wich
cart you like to keep. Despite the authority of Raul
(I admire the guy immensely) the only cart that is
compatible with the Technics is the Musik Maker.
My personal preference is FR-64 s (to keep) but that
is not an advice. Even despite the fact that all other carts are comp. with FR-64s. BTW there is some kind of luxury in your 'dillema'.

Cheers

>>The Hadcock is a low mass arm IIRC and is supposed to be an excellent match for the Music Maker, which makes sense because the two are made by the same manufacturer.<<

Let's clear up some bad information here.

The Hadcock effective mass is about 10 grams which is actually intermediate using the writer's own criteria.

Secondly, the Music Maker is built by Leonard Gregory, aka The Cartridge Man.

Hadcock tonearms are owned and built by Charles Hadcock who assumed the duties when the founder, George his father, passed away a couple years ago.

The companies are two separate and distinct entitites.

Dealer disclaimer

Dear Nandric: Do you use/know in deep ( sometime/overtime ) that Technics tonearm?

well, I still own it and comparing with the FR's/Hadcock IMHO the Technics is a better choice.
If you never try it my advise is that the next time you see it buy it, you will be surprised how high quality performance show against other top tonearms either vintage or today ones.

The EPA-500 was designed/build on the Technics " golden years " along the EPA-100 and EPA-100MK2 ( as the TT SP-10's ), I admire FR but the Technics on those " days " build a " little " better products, at least is what I already experienced with all those products.

Anyway mine is only an additional opinion and of course like always our preferences are system/ears/own-priorities dependent.

Regards and enjoy the music.
Raul.

"The companies are two separate and distinct entitites"

This is not a totally true statement, as Leonard Gregory has "exclusive" marketing rights to the Hadcock company.
There is no other "contact" for Hadcock, and to add even more information, the modern design improvements of the Hadcock tonearms were done by Leonard Gregory, which is how he got the exclusive rights.