According to the AQ-65 manual, it is not compatible with 48's with serial numbers 110,000 or less. It is compatible with serial numbers 110,001-110,050 "w/modification" (not sure what that is), and "fully compatible" with serial numbers 110,051 and above. My machine's serial number is lower than 110,000 and the AQ-65 works great...not sure how! Nevertheless, this supports your idea that something changed in the 48. The odd thing is that I bought my Tascam 48 manual directly from Tascam about 2 months ago. They didn't ask about my serial number (which is under 110,000). The manual they sent contains the older schematic. Maybe the higher number machines are rare.