Curved and Straight Tonearms

Wow.....

Nandric,
There is a lot of myth in audio. The most common theory is that S shaped arms evolved from J shape to place the horizontal centre of mass of the arm tube/cartridge perpendicular to the 2 horizontal bearing points at the arm pivot closer to the centre. If you look at the J shape it places a high mass further to the inside and loads up the inside vertical bearing relative to the outside vertical bearing. As you lower the cartridge onto the record with a J shaped arm it will try and lift the inside bearing. With knife edge bearings such as the early J shaped SME's this means the inside bearing is unstable. Lateral balances are provided in some vintage arms to help correct the offset centre of mass of the arm tube/cartridge.  

Straight arm tube arms are more common now, but a key point is that along with straight arm tubes most modern gimbal bearing arms now have offset bearings; that is, the vertical bearings at the tonearm pivot have an offset angle that matches the optimum offset angle of the cartridge determined by the pivot to stylus distance.

Most vintage arms including the FR64S do not have offset bearings.
The disadvantage of non offset bearings is that when the arm goes up and down a rotational force ( twisting ) is applied to the cantilever - the use of a lateral balance never eliminates this effect, but it can reduce it somewhat.

The lateral balance on the FR64S has multiple implications, because apart from providing adjustability to the inside cancelling force, the distribution of mass around the bearing housing alters the loads on the bearings, and ultimately the forces, both rotational and lateral, on the cantilever as the arm moves up and down and back and forth on eccentric records.

For this reason rather than second guess ALL the engineering considerations that have gone into the design of the arm, one should set an arm up as per the manual in the first instance. We have a saying "a bad workman blames his tools" - this is so very true of poor tonearm and/or cartridge set up that all too often leads to indifferent results. 

     
     

I would not want to be one of those who recently paid a premium for the FR66S, however, where the asking prices are up around $6-7,000.

I’m not sure if you are overestimating Raul’s influence Lewm (if I am understanding you correctly)...in suggesting that the prices of the FR-66s arms have gone down since other folk paid a ’premium’ for theirs...?
I have two FR-66s arms (as well as a silver-wired FR-64s) and have seen their prices stabilise at about $9000-$10,000 over the last few years...
http://www.topclassaudio.com/web/eng/used_product_details.jsp?gid=8068
That’s if you can find any available at all......
There is a badly beaten-up one available with missing items and without headshells for $5,850 from Otoman Vintage
http://www.ebay.com.au/itm/352099982219?rmvSB=true
but this is hardly the quality that a serious collector or connoisseur will contemplate.
At any rate....analogue-lovers willing to pay these prices for 35 year-old tonearms will do so knowing what these arms can do,
And what they can do, compared to the very best ’modern’ arms....is still quite baffling!