Chuck, I agree with you. External analog TGC circuitry is much more convenient and flexible. And it's cheap!! Schematic is very simple:
[RAM(TGC curve)]=>DAC=>Integrating Amp.
8-bit DAC is enough even for a very sophisticated systems. DAC08 or any clone of it could be used, followed by any clone of TL08x.
And once again... For everyone who 'plays' with ultrasound: Multilayer PCB(6+ layers are welcome), digital LVDS signals should be routed close (+/-) with the equal length.
Trust me, the PCB design has a very big influence on the image quality.
Regards,
Mariusz