Hi Toocan,
Hope my comments help, I spent a good while trailing all the forums and websites, and actually got a very good response from Kevin Green at Audiocellar (
www.audiocellar.co.uk)
To quote from my correspondence with him.
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Early amps - mk1 - grey, five inputs.
Mid amps, - mk2 - black, sometimes with a silver top cover (1991 only), six inputs
Late amps - mk3 - all black, PSU changed to remove earth and double insulate
Serials with B,D,E,F are the easiest to support going forwards, before this there are now many obsolete things that are a pain.
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I personally had a mk1 (grey, on the back you will notice it has separate MM/MC and Load inputs), the newer ones didn't have the load connectors.
I will be getting my mkIII in a few weeks times. Looking forward to some nostalgic times with it
.
Also, other extra and useful information I found was from this post too
http://www.avforums.com/forums/audio-processors-power-amps/217871-audiolab-8000a.html
Let us know if you do get one, they are still very good even after 10/15/20 years.
Cheers
Chris