Hi Raj:
I saw your question last night (before there were any replies) and I was going to answer it then, but apparently just at that moment, there were some problems with this site's server's and I couldnt post the message.
Anyway, The DD/DC stands for Diamond Differential/Direct Coupled. Supposedly, the "Diamond" terminology comes from the idea that when the circuit is drawn out on paper, it resembles the shape of a diamond. The DC refers to not using coupling capacitors in the signal path from the power amp section to the speakers, for what is theoretically claimed to then offer purer audio sound quality. If ALL coupling capacitors are eliminated, the DC is then said to be a reference to the terminology known as 'direct current' amplifier and the amp can uniformly amplify down to DC (O hz frequency).
Linear-A refers to the particular design used in the AU-D7 (and AU-D5 models). Since Class A amplifier designs tend to be inefficient because their transistors are always on, it eats up alot of power whereas Class B designed transistors do switch on & off but in doing so, while they do save power and minimize heat gain, give rise to large amounts of switching & crossover distortion. Thus, Linear-A was Sansui's design used in these models which combines the advantages of class A & B, but without their disadvantages.
The bigger, more powerful models in the AU-D Line, AU-D11 and AU-D9, are different from the D7 & D5, and use a circuit design called "super feed forward", its a sortve play on words, instead of feed-back, its feed->forward
As for specs on the AU-D7, a service manual is suited more toward repair, though there are specs contained therein. The AU-D7 brochure (or the D5 brochure) would be the best source of explaining the amplifiers features and would also provide the specs.
I have an AU-D7 by the way. And yes, it was offered in two color options, black and silver, and the same applies to the matching tuners, the TU-S7 and TU-S5.
B/F