Quad Mk I has straight edges (on top sides) and Mark II has bevelled edges. If you can identify by this method good, otherwise I will get you some more information if you like about the differences between MkI and MkII which I believe I did save somewhere on my disk long time ago. When I get home I will check it out. Let me know if you really need the info.
Following is the information I had saved from the post in mailing lists that I subscribe to long time ago, I hope it is of help to you.
The Quad 606 mkI has a totally different sound to Quad 606mkII. The 606mkII was a major improvement upgrade, having a totally different style of mains transformer, increased value smoothing capacitors and other circuit changes too. The Quad 606 mkII has the top corners of the case "beveled at 45 degrees" and is above s/no 21,000. Although several hundred 606 mkII 's escaped in the old cases to use up stock on hand. If the s/n is above 19,500 take the case off and see if it has a torrodial mains transformer (big round thingy), and if so, its a 606mkII.
Alan Shaw of Harbeth uses a Quad 606 as a reference amp, with his daughter voice from tape as source, when designing his speakers. And some of his speakers are exceptionally well regarded as well as being broadcast monitors. It can't be all bad. And Quad gears hold its value well, so you could try it and then sell it again if not satisfied.