aoTuV is an abbreviation for Aoyumi's Tuned Vorbis; it is third-party development/tuning of the Vorbis encoder.
aoTuV versions improves significantly on Vorbis quality: Most people agree aoTuV beta 4 (and newer) achieves transparency at -q 5.
Released in December 2005, aoTuV beta 4.51 improved further on low bit-rate and after peer review was rebranded aoTuV Release 1 with some reports that -q 1 (approximately 80 kbps) is good enough for streaming.
In June 2007, the aoTuV beta 5 versions including 5.7 underwent peer review and superceded Release 1 as the HA recommended Vorbis encoders, improving the low bit-rate quality in relation to Noise normalization without sacrificing compression ratio.
aoTuV Beta 6 versions released in 2011 made further improvements on pre-echo and post-echo handling, stereo mode decisions and noise normalization at low bitrates but have not been extensively peer-reviewed by the HydrogenAudio community.
See Recommended Ogg Vorbis page for more details.