Becareful with that Phlegm.
As I'm sure you are aware, mp3s use lossy encoding, meaning that when the files are decoded, they are not exact copies of the original.
So in making your 128kbps bitrate mp3s from 320kbps sources, your source file is a lossy decoding that is a lower quality than the original digital source. It's like making a dub of a dub of a cassette tape. Your 128kbps mp3 won't sound as good as a 128kbps mp3 from a CD audio source.
Likewise, transcoding wma to mp3 or iTunes to mp3 will have the same problem. While 128kbps mp3s from CD audio sources sound fine, anytime you trancode from a lossy file format to mp3, you'll want to use a fairly bitrate to minimize the loss in sound quality.