sound quality on iPod Classic through car speakers

Hello. I have two iPods. One is an iPod Touch (32GB, 2nd Generation I think), and the other is an iPod Classic (128GB). I bought the iPod Classic after I filled up all 32GB of space on my iPod Touch (they hadn't released the 64GB iPod Touch at the time, so I bought the 128GB iPod Classic). Most my favorite music is still on the iPod Touch, so that's what I use most, and it works and sounds absolutely great; however, some of my music is on the iPod Classic, and it doesn't sound as good as the iPod Touch. For a few songs, I have the same song on both iPods, and the song sounds very significantly better through the car speakers using the iPod Touch than it does using the iPod Classic. I have a Ford Explorer that has a stereo input on the center console, so I listen to my iPods through that input with a standard stereo cable (I don't have to use one of the radio tuner adapters or anything like that), and the iPod Classic's audio quality is very noticeably inferior to the iPod Touch's audio quality.

So, I guess what I'm asking is, has anyone else experienced this? Is it normal that the iPod Classic should sound inferior to the iPod Touch? The difference is clear, so the only other thing I can think of is that maybe my particular car's sound system doesn't respond as well to the iPod Classic as it does to the iPod Touch, but I don't think that's likely. I guess I could test this by listening to both iPods through a different source (like headphones), and I'll do that, but in the meanwhile I just wanted to check and see if this is a common problem.

Thank in advance for any help.