Any views on the respective quality of all-in-one systems (e.g. Onkyo CS515, Denon M37) and low-end separates (from, say, NAD C515BEE to Marantz CD6002 and equivalent range of amps) - i.e. in the £250-400 price range (excluding speakers)?
Either has their place, and at that price point I'd be happy with either, probably - my old Denon was 'proper' hifi with half-decent speakers. So it's down to how you want it to look, and how upgradable you want it to be, IMO.