AA4T are not the BH-Quality ics. BH-die are the 1A4T ics. AA4T are CH-quality. Regarding availibility, you'll have a tough time finding either in the US at this point. There are still some places selling the 1A4T but I don't believe TwinMOS is using the CH-die ics any more.
Good stuff though as I have both the AA4T and 1A4T gigs.