Both the Sound Devices and the Aerco MP2 pre-amps are excellent pres. No mods. desireable or necessary.
A>D is the conversion to digital of the original analog signal. The iRiver does not have a reputation for having a very high quality conversion, so most users prefer to feed it a digital vs. analog signal so it only operates as a 'bit bucket' to capture the digital data already converted by something else in front of the recorder in the signal chain.
Since you already have an iRiver, this set-up will work fine for you and you will use the stereo mini 1/8" tape out of the mp2 to feed the analog line-in input of your iRiver. Eventually, however, you'll prob. want to upgrade to a better (and 24 bit capable) recorder that will have superior A>D conversion that the iRiver.
Here is an example with almost the same set-up:
http://www.archive.org/details/dnove2006-12-01.akg451s.flac16AKG c451b/st -> SD MP2 -> Edirol R-09