why does preamp improve sound quality?

Harleys book is what several members of gon recommended to me as I was getting back in the field - I found it very useful admittedly not so much for the nitty gritty electronic analysis - as more general overview of each components issues and what to look for in putting together a system

One thing that no-one seems to be mentioning is that the line section of a preamp also has to control the interconnect cable. The better they do this is often also part of the measure of quality a preamp can portray. Passive controls do not control the cable at all (hence the difference in sound from low to high volume) so if a preamp is effective at controlling the cable *and* is competent at the other tasks of a line section (low distortion, wide bandwidth, etc.) then it can *easily* sound better then a passive.

Some might look at this as an impedance issue, and to a large degree it is, but it is not to do with how the unit drives the amplifier so much as whether or not the construction issues of the interconnect are adequately bypassed enough so as to be minimized.

And mind you, not all line sections are up to this task. If not, a passive will be better...

Thank you for posting, Ralph!

I, for one, understand the preamp role better from having read your comments.

I agree, thanks for the post! Could you explain how the preamp controls the interconnect? Thanks in advance.

An attenuator (passive) doesn't control anything but the volume function (voltage usually). In a passive attenuator, it's the source component that effectively drives the load via the volume control + connecting wires. If one has a source with a strong output stage &, say, a compatible impedance TVC (or a carefully designed ladder) there should be no problem in driving the load without an active stage inbetween...
Of course, change the source & you may have problems.