What's the story?
Possibly the most famous amplifier in the history of hi-fi, the Quad II was such a success that its manufacturer changed name – Acoustical Manufacturing Company became Quad. It's actually a mono power amp, which means you need two for stereo – one for each channel – and a preamp to control volume and source selection (as if you'll want to plug in anything other than a turntable).
Why should I want one?
Because it's the quintessential valve amplifier, with a look that's unmistakably post-war. It's like a mad scientist has taken an angle-grinder to a T-Bird and screwed some lightbulbs into the top. It's also still capable of mesmerising sonics when partnered with the right equipment.
What to look for
A lot of old Quads have been modified over the years and they generally sound worse, so try to find an unmolested pair that have serial numbers not too far apart. You might need to replace the valves and get someone to fit phono sockets. If you want the look and sound without the hassle, Quad now makes the II Classic, a faithful reproduction. Sadly, they cost £2400 each. Either way, because of their low power output, you'll have to be careful to match them to sensitive speakers.