A Quad service will actually check all the components which are known to degrade or drift. The good thing about Quad is they do not change stuff for the sake of changing it, if its still within spec it stays untouched. It will also still sound like a 606 when its been done too which I think is a bonus as I like the sound the 606 makes.
Analogue
401 / PU7 / AT33ev
Longdog Audio Quartz turntable PSU
Longdog Audio MCJ1 - Prototype Phonostage
Digital
Innuos Zen Mini Mk 3
Topping D50s
Caiman Seg & Dorado
Cambridge CXC
Amps
Tron Seven Linestage
Bel Canto Set 40 or Nick Gorham built 300B Set Monoblocks
Speakers
Tannoy GRF Memory.
Tannoy ST100.