This would provide no insight.

What I think you're saying is that you want to measure frequency response, which is a question that's just off the mark in many ways. First of all, frequency response is a product of everything in the chain, definately not just the source. Besides, plugging a source directly into a recording device is nonsensical; think about it: it's the equivalent of trying to judge a camera's colour reproduction while looking through a filter of random colours. Measuring frequency response is not something you can really do well at home, even with high-end recording equipment. But all that is moot, since no piece of consumer technology aims to give flat/balanced frequency response (that is, not boosting some frequencies over others). This is the chief concern for audio recording and mastering, for which unembellished reproduction is needed, but it absolutely is not for consumer products--even the very high end.

Lastly, you shouldn't be worried about the sound quality if what you're doing is plugging a portable device into a stereo via 1/8" for two reasons. First, this is the worst way to transmit an audio signal, but it's all relative so if you consider sound you've heard over 1/8" to be "crystal clear" then we've just got different standards. Second, almost every MP3 player or PDA out there has very similar specs for their amps, which are by no means sophisticated or great. Maybe a few devices have noticeably better quality and a few noticeably worse, but this is again completely subjective since it's not the aim of consumer audio to give balanced frequency response. Play around with your EQ on any device, and you'll probably find something that sounds "better," but it's certainly not more accurate.