hi,
i've used mp3 for a long time now, and have recently shifted to using vorbis. and does it rock or what!!!
i use q4 - q6 most of the time and the quality of music files is really impressive.
anyway i noticed that at low bitrates the high freq sounds were getting boosted. does this happen or am i just hallucinating?

It's a known issue, e.g. discussed in threads related to rjamorim's latest 64kbps listening test.

Yep it is known issue and Monty is going to try and fix that in Vorbis 1.1, so I've heard.