That's a different story.... For MP3s, unless the majority of them are 192K or better, I wouldn't spend extra cash on better speakers. However, you can always use the amp/speakers with a CD player as well, so it comes down to how much you really want to spend and how likely you are to use these speakers for something else down the road. IMO, the M3s are no more pleasing to the eye than the 170s. At first, I thought the 170s were kind of plain and unattractive, but now I kind of like their small size and simple appearance.
One more you might look into would be the Mordaunt Short 902 or 912 from
www.ac4l.com They aren't as smooth in the midrange as the 170s, but they give a very good overall presentation and are fairly neutral (a tiny bit of metalic coloring to the midrange due to the aluminum driver). I find my 902s very pleasing with vocals and soft music, but they are equally at home with rock and jazz.