Well, the thing is, it's probably using a full 96 kbps for the one channel. Since 128kbps probabyl sounds fine for FM talk radio, try encoding at 64kbps mono CBR and see if it sounds okay. If you thought you could get by at 96, then try 48, too. There's also a 56.

OTOH, ISTR a bug report that the empeg wouldn't do mono files properly. You might want to test one before you go hog wild.