Since memory and processing power are much more affordable today, there's no technological reason why sound quality should go down. That being said, the G-70 was a TOTL board and technology was ALREADY quite advanced by 2006. So any further improvements since then are incremental.

Where you really see it is with the low-end boards, which now sound almost as good as TOTL from years ago. Listen to some demos of Casio AiX, or Jeremy See's comparison between the Roland X-30 (not a real Roland BTW) and the Yamaha EW-410 (Ew! I know ;-) Don't like the feature sets on these, but they both sound good.

If the sound quality seems worse on today's TOTL, I suspect it has to do with effects, mastering, and EQ (all of which are user-adjustable), not the base sound samples or the DAC. My $.02