Audio Research D79C Amp

Hello,
Recently I acquired for $1800 an Audio Research D 79C amp in really excellent condition, I would say a 9 out of 10.  It seems to have had very little use.  I had it checked out by a really good technician and brought up to spec and it really sounds gorgeous.  I am using it with an Audio Research Sp10 Mk II preamp into Proc Ac Response 4 Speakers. I know that we all hear reproduced music differently, and I tend to prefer the older Audio Research sound and am very happy with this combination.  I have found very little information about the series D79 amps and am wondering if some ot you experts on Audio Research could tell me a little more about this amp and where it fits into the D series of amps by the company. In the owner´s manual, the date on the schematics is November 1982, so I am assuming that the amp was probably built in 1983.  What I would also like to know is why for a 75 watt amp, the unit is so lage and heavy.  It weighs 85 pounds net.  Any further information or comments on its use would be most appreciated.
Cheers and many thanks in advance to all.