Audible difference in bluetooth A2DP sound quality - how change A2DP settings?
Looking into this problem further, I also found a google code issue about it. The discussion there points out that the problem seems caused by the bitpool issue, and is fixed in CyanogenMod (at least the latest versions). Also, even stock 2.3 is reported to improve the situation slightly. 

So switching to CyanogenMod seems to be one (and only?) way of resolving the issue.