I've always preferred to get all gain from the church pre and not from the iRiver.... so 0dB there for me and then 11:00 on the 9100 which is unity (0dB) and then go up on the pre... If that 20dB gain isn't enough, only then add gain on the iRiver up to 24dB. After that is digital gain, same as raising in post.
The REAL issue here is that the bass sounds like crap! Right? Mids and Highs are sort of ok sounding, to me.
From my past experience, your pre-amp battery is low but not dead. Get out your Digital Multi-Meter (DMM) (Here is one for less than $3us, less than a battery, don't pay more than $10
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=98025) and check the voltage of the battery you made your sample with..... I'm betting that it is BELOW 7.99 volts. 7.5 and lower is when I got the mushy bass tone heard here.... My pre (now sold) was version 2 and yours may be a newer version, hence the different minimum voltage. Later versions put out more plug in mic power, around 9 versus the 4.5ish of v2 pre-amps.
Good luck,
Joe