Will a high quality preamp really make a difference in sound?? Also speaker cable

Any additional processing in the signal path will negatively affect the signal, including a pre amp.  Do you need additional amplification, or does your source component(s) have sufficient output to drive your amp?

If you already have sufficient signal from the source component, all you are doing in a preamp is amplifying the signal, and then attenuating it - amplification will cause additional distortion, and the attenuation will waste part, or much, of the signal.

There are passive preamps that only waste part of the signal, or TVC preamps that do not waste any of the input signal.

If you are looking for tone control in a preamp, the go for it. That might represent improved sound.  Depends on what your objective is.

Many seem to think that the more fancy equipment in the rack, the better the stereo sounds.  Not so much the case....

The preamp can be the most colorful and important part of a system.  Will you personally like having X preamp that cost N number of dollars? I have no idea.

My dealer (whose opinion I respect) thinks the pre-amp is more important than the amplifier, for whatever that is worth.

Additional processing will not necessarily negatively affect the signal. This is a misconception. And at the very least, positives may outweigh negatives.

Preamp without a doubt is the most important