This is just plain lazy on Ampeg/Yamaha's part. Although I have and love the SGT DI, I don't see the merit of releasing a class D pairing when the premap is designed to work with most power amps. The incentive should've been the inclusion of additional popular bass and guitar models and the inclusion of tube in the preamp section.
How is it lazy? They have taken their new platform and put it into a lightweight amp. To me that seems pretty logical. How many bass players do you know of who will do the late 70s-early80s thing of pairing up pre and power into a rack these days?
.