why does preamp improve sound quality?

I have tried going direct several times but having the preamp in the chain has always been better. Arthur

A good preamp can improve the impedance matching between sourced and amp, and also give the music more body and life, primarily from extra gain. I've tried it both ways and have preferred a good preamp every time.

I'm sorry if I'm not as clear on how impedance and other factors affect your situation. I've always thought that introducing the gain of the preamp between the two means that the amplifier has more power in reserve to accurately transmit the signal, with particularly better dynamics.

There would have to be more distortion introduced with the extra stage, but it sounds like there is more distortion when your power amp takes the whole load itself.

Elizabeth, you addressed the issue of impedance mismatch between the source and poweramp. Can you explain what kind of impedance mismatch? In terms of electronics, what does a preamp do with the signal before it enters the poweramp?

It probably has to do with matching the output impedance of the source with the input impedance of the amplifier. If you have all of the manuals, find the specifications for:

Mark Levinson no. 390s "output impedance"

Quad preamp "input impedance"
Quad preamp "output impedance"

Quad II-forty five amp "input impedance".

After looking at these, see if the numbers suggest that "CD to AMP" would be better matched with the preamp inline.