When using a DAC does the cd player really matter?

I tend to agree. Provided the player has a solid, stable transport mechanism and you're happy with the remote control and functionality then you're set. That's why a lot of people look for older players like the Pioneer Elite models that don't sound great by today's standards from their analog outputs but have the famous "stable platter" transport as well as rock-solid build quality. But I've also used $50 DVD players that sounded as good (through the DAC) as much more expensive, dedicated CD players. Your mileage may vary.

That said, what's your budget? If you're planning to spend $1000 or $1500 total for example I have to wonder why you would want to go through the hassle and complication of two boxes, two power cords and one digital interconnect when $1000-$1500 players like the Rega Apollo and Cambridge Audio Azur 840 provide digital playback that's within shouting distance of "as good as it gets."

get a good or very good dac then either look for a decent transport or use your computer as a transport. imo, the dac makes the biggest difference as long as your transport is good.

Assuming you use an external DAC, the biggest need is for the transport to accurately create a digital stream. Not all transports are created equal. Quality of the physical components, good circuit design, noise free power supply, and the quality of the clock all impact the ability of a transport to generate an accurate, jitter free stream. The more revealing your system, the more it will reveal transport limitations.

This is the essence of why so many people (myself included) are fans of file based transport (Sonos, Slim Devices, Computer based) instead of a disk based one. It is a newer approach, and as such takes some doing to implement well, but dollar to dollar you can get a lot better performance in my opinion.

why is a "file-based transport" system better? Don't you have to use a transport - or worse, a computer cd drive - to read the digital data into the files, wso the file then contains all the same errors? In other words, the file is no better than the transport/cd drive you used in the first place?

IMHO the DAC is much of the sound presentation and much more important that the transport