One quick Q: you say you had the gain to 40, but did you have the setting on external mic? You have to select that rather than "line in". (obvious, I know, but it's actually an easy mistake to make if you're new to the iRiver).
I don't know those mics, so can't comment there. I have run AT831's and CMC8's into my iRiver, and found no problem at all with gain. The only mic I tried with it that wasn't sensitive enough was a Sony ECM907 (that I had used w/my Minidisc)
If you find you DO need a preamp, I have a Reactive BoostBox that I use with my JB3, and I've found it to be a decent little performer if not in the league of the AD20 or other more 'pro" pieces of gear. It does the job just fine, however, and it's very easy to use and not that expensive.
Also, have you tried running your iRiver w/Rockbox firmware - you'll be VERY glad you did!! It's much MUCH better for recording w/features including great level meters.