The late production Quad 606mk1, which was basically the mk2 without the styling change and onwards, were excellent amps which should all 'sound' the same!' Our first dem early production 606 needed a couple of hours from cold to remove some 'grain' from the sound, which was audible and predictable and something that didn't exist in all other samples I ever heard, the amps 'coming on song' pretty much from switch-on and staying that way thereafter, which is what any amp worth its salt should do.
There was one early batch of far eastern made 909's which for whatever reason, had less than idea electrolytic caps fitted, causing noise and audible distortion I gather. There's an entire tale as regards how these caps were produced over the computer industry and so on and once this was dealt with, I've heard of no reliability or sonic issues at all! It was only the electrolytics used I believe!
The current Artera version has I believe, a slightly wider bandwidth according to one review I read. No idea if this is audible at all (I very much doubt it with most speakers), but the only criticism I could remotely make of it is that it's not a *showy* looking thing. With matching preamp, it's lovely and can drive all manner of speakers without stress (the preamp is a luxury thing at a not 'luxury' domestic price and I'd have one like a shot). My local audio salon appears to have dropped the brand, as it sat in the cubic display unit with their HiFiMan and deluxe Grado headphones, deluxe catalogues and so on and seemed totally anonymous when acres of Naim, Chord and now Accuphase is set up nearby in racks costing many hundreds per tier, none of it really any 'better' at all, but sure as heck making a statement, financially as well as visually. My photography is terrible, but spot the Artera bits in this display rack - I wish Quad had adopted the 500 series pro amps stylee, which look naughty in the flesh even switched off
View attachment 390435The Quad bus enabled balanced connections, but I also read of bypassing the bus. Quad's service dept may be helpful as they do full refurbs on old gear now, not simple repairs (a pal sent back a 34/FM4/405-2 and they really went to town on all three items, even to the point of a new memory chip in the tuner adding extra presets apparently.