Fidelity Research FR-64x

I have the same set up as you. In my experience there is a clear sonic gain with the N-60 stabilizer nut. The added mass seems to benefits the 'bottom end' of the sonic spectrum and makes the whole presentation somewhat more relaxed. This can be observed with both the 64s and 64fx arms. Sonically the B-60 base does the exact same thing, with the added convenience of VTA on the fly.

If anyone is interested, I have a spare N-60. It's a replica, but then again this is just a piece of machined stainless steel. No washer, but the same weight and same dimensions as the original. The only difference is that the original has a sticker with the model number.

My love to Ikeda & FR tonearms getting stronger.
I'm a junkie about NOS vintage gear, i couldn't ignore Fidelity-Research FR-64fx in NOS condition in original box with documents (complete set). What a wonderful tonearm! Arrived this week, this is really a state of the art design. I like its black finishing, each time i compare it to the FR-64s i like the 64fx better. I've been using another sample with W-250 counterweight for FR-7fz. It's funny, but looking just for a small counterweight i ended up with another sample of FR-64fx (NOS) with this counterweight. So now i can use this arm with conventional cartridges (higher compliance). 

Wonderful. The general consensus is that the S is superior (of course cart compliance is a factor) but this is yet again proof that there is more than one answer to most audio questions ;)
I only have the FR64S so can't compare...