Brian,
for the last decades McIntosh has always replaced an old model with a slightly more powerful, by renaming the newer one one number higher - e.g. the MA6400 became the 6450, than the MA6500, the MA6800 was followed first by the MA6850, then the MA6900. The MC352 became the 402. The C100 preamp became the C200, followed by the C1000 and C500.
Following this logic, the MA6500 is replaced by the MA6600, the MA6900 is replaced by the MA7000.
After a look at all new models, it seems that McIntosh has changed things a little bit, as indeed the 6600 has autoformers, while the 6500 had not. However, the 6600 has the same bass and treble controls like the 6500 had. It does not have the elaborate five-band-equalizer the 6900 had, and which the 7000 has, too.
The 6300 is no replacement for the 6500, but rather a new model beneath the so far two integrateds - McIntosh product manager Ron Cornelius said as much on a post over on audiokharma, if I am not mistaken. The 6500,which I have owned, offered 200 watts at 4 ohms, whereas the new MA6300 offers only 160 watts.
Regards,
Florian Hassel