I also have the standard speakers. I know that the iDrive isn't supposed to cope with high bit rate files, but it plays my huge lossless files without problems. Generally I have 320 AACs, but some 256s and some lossless files.
All play well, and all (even the few 128 MP3 files I have) sound significantly better than the iPod with the same files on. Maybe I have just developed too critical an ear which spoils listening to bad quality music
The standard speakers aren't as good as my A3 standard speakers (which had tweeters and a sub, although I had to tone down the sub as it was far too boomy), but they are a lot better than I expected after reading reviews.
Has anyone tried the Alpine upgrade on the F20 yet?