The answer to your question is 'it depends', largely on how much circuitry there is between the line input and the pre-out.
Years ago Rotel sold what was IMO a scandalously expensive so-called 'control amplifier' (RCH-10) which was basically a set of line inputs fed to a source selector and a volume pot. It was on sale for a stupidly high price to the rich/gullible who didn't realise (or maybe didn't care) they could build the same thing for about £10 using readily-available components at Maplins. However on the plus side, because there was nothing in the signal chain other than the source selector and the volume pot, you could easily argue that its affect on the sound quality of the incoming sources was knocking on the door of nothing.
Modern amps, especially amps with digital controls, are far more complicated, with potentially more circuitry in the path between line-in and pre-out. Vlad is spot on with regards to vinyl, the amps with the best phono stages probably don't come cheap, but with line level sources, it's all about the quality and/or complexity of the internal circuitry. And I'm not convinced there's definitely a link between price and performance, seeing it's still very possible to build your own passive 'control amplifier' on the cheap consisting of just a stereo pot, RCA sockets and a source selector, which will be as acoustically transparent as anything available.