The FSC and the HD650 sounds similar. To me it is the closest headphone to be considered a closed version of the HD650. From 200Hz/300Hz to 8KHz they are almost the same.
It (FSC) has rich, upfront, full bodied and beautiful mids, pretty similar to the HD650 ones. Also, it has the advantages and drawbacks of a closed can. If has better bass response, more defined and detailed, with an extremely flat response to the lowest end. The sub bass is more present although it has a bit less mid bass emphasis, maybe more due to of its flat response in the bass region than to a lack of mid bass. The highs are gentle but are not rolled off. Obviously, much more isolation.
But it has the disadvantages of a closed can. Closed sound. Significatively narrower and closed soundstage with less air. It is good for a closed can of its size, but it is not near the level of the Senn or Denons. In the midrange the FSC is very good but I think it is not as detailed as HD650, which is more textured and have more microdetails. In the highs I think the HD650 has a bit more extension (I mean extension to the top end, not volume) and sense of air. Also, the FSC is more unconfortable.
But you can't have all in only one headphone.
On other post I have made a comparison between FSC and FSP so you can get the idea. In summary, the FSP is a neutral version of the FSC, with less bass pressence and more detailed but drier and less full-bodied mids. I like the FSC more.
I hope that helps.
