ABXing is comparing two files (A and B) to the reference X. If you can repeatedly successfully identify whether A or B is X, the files have a great discrepancy in quality. Increase the bitrate until you can't reliably ABX it with the one before it.
By the way, "-V2 --vbr-new" is a good recommendation, but "-V2 --vbr-new -q0" is of higher quality. Both the speed and the quality increase is small. The default is "-q5".
EDIT: Alternately, you could use "--preset fast standard -q0". And yes, "-q0" is a pet peeve of mine
UED77
