,,,what matters is the quality of the parts
True, but it’s worth noting that if you took the preamp section out of a good integrated amp, gave it it’s own power supply and enclosure, and used it to power the same amp, it would have some benefits over the integrated version of the same components. How much of a difference that can be heard is another variable by itself, but there's definitely that potential.
There’s never a free lunch though...separates require a separate set of RCA connectors, which can introduce a new source for degradation if not done well. The good news is that it’s not hard to do a competent job of picking that cable.