Does the FR2LE have the combo XLR + 1/4" jacks? If so, you might be better off going from the RCA's on the MX-100 to standard 1/4" Tip/sleeve into the FR2LE.
(assuming you don't have a modded unit which sometimes disables them)
If you read that Rane page
http://www.rane.com/note110.html is says pretty clearly:
- avoid doing a unbalanced <-> balanced conversion if you can.
- if you must do a conversion, use an isolation transformer.
-
If transformer isolation is not an option, special cable assemblies are a last resort. (their bold italicized emphasis).
You can buy a simple RCA > 1/4" cable at Radio Shack fairly cheap. Why jump right to the method of last resort?
There is no inherent advantage to jumping to XLRs, unless you are running long runs between the MX100 and the Fostex... at which point you should use transformers.
Also, if you accidentally turn on the phantom power to the Fostex you won't send it back through the MX100.