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....

If a preamp is really all upside and no downside, as contended here by some, then the truly high end systems should have a couple of preamps daisy chained to get the best possible sound.  Odd that you never see that, even when funding is no issue.

Any time a signal is processed / amplified it is changed, unless you assume the amplifier is absolutely perfect.  Of course that is never the case.

If you have a deficiency in you system, like you need to add some tube warmth, or tone controls are required to balance the sound, then a preamp is potentially in order.  But note that the effect of the preamp is to alter the sound to achieve some end. 

I am not suggesting that there is no place for a preamp, but useful to be aware of what it is doing, both good and bad.