There's a point at which I can hear a huge difference between a bitrate and the next bitrate. This is usually between 96 and 112. But I think that the difference is not the bitrate itself as much as the fact that this is the point where most encoders decide to resample down to 32kHz sample frequencies, and they seem to do a poor job of it, if it's even really possible to do a good job. See if you can force your encoder to use 44.1 kHz at these low bitrates. Not that this has much to do with the original question, as the problems at these bitrates are usually severe enough, regardless of resampling, to be apparent to even the most tin ears.