£550 is an excellent price for a FULLY SERVICEABLE power amp which is happy with 4 ohm loads. Quad's service dept now do updates to older stuff (a pal had his 34/FM4/405-2 fully 'done' in Huntingdon recently and they realy went to town on them, surprisingly so, upping the FM4 to extra presets as well as the usual memory battery and recapping the 34 (can't remember what was done to the 405-2 but it was similarly refreshed!
Quad 606/707/909 family of amps are great and now traditional type models. HiFi World (now defunct), which was a valve/vinyl/Tannoy speaker kind of place for years, tested some power amps and were very pleasantly surprised by the bench tests and also of course the 'sonics' of this model. When I can, I'll dig the review/measurements out and scan them.
The very first 606s took a couple of hours to 'warm up' and lose a little 'grain' we all heard up top, but this was soon sorted and the later breeze-block 606s were pretty much 'there' right from cold starts. late 606s and the chamferred 606mk2, had a toroidal mains transformer and a 50% increase in supply capacity (twin 10,000uF became twin 15,000uF) which I believe were longer lasting than the leaky 405 supply caps.
The 707 had then deeply unfashionable and unknown in domestic audio 'Speakon' connectors, which have lately been shown to be superior to 4mm plugs or spade connectors. Superb amp, but no idea if anything was changed in the circuit.
The 909 had the Quadbus input and here may be a subject for discussion. Early Chinese made models had sub-standard caps, but these should have been dealt with by now. All subsequent production were fine, as was the QSP and the superb and still reasonably priced (compared to the overpriced bling-tat some of our high end dealers sell) Artera Stereo I love so much - the Artera preamp's nice too and superbly built and presented I think. The Artyera pair on dem at my local audio salon all but disappeared in a display system sat alongside acres of 'Fraimed Naim boxes' and they now don't seem to sell the brand, which obviously isn't as profitable as the other stuff they promote at multiples the price (but I'd suggest not multiples the performance).
Does that help? Once my 'Linn/Naim-chip' had been extracted in the late 80s and I woke up and started looking around with fresh eyes and 'ears,' I was amazed how capable this Quad block of a thing actually is, as it'll drive 230WPC or so into 4 ohms without current limiting or cooking to death as the 405s do. Sure the modern whizz-BANG chip amps raved over here will do better on the bench and Class D has finally come of age, but the 606 family of Quad power amps will be running I suspect, long after the others are in landfill!