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

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 

Clearly you should seek out the best engineering and scientific perspectives on this question which means you should choose a pre-amp that delivers exceptional measureable performance. There are so many exceptional performers out there. You can consult resources like Audio Science Review to learn more about amplification and audible thresholds. There are also useful notes on source switching noise and power-on behavior.

Professional engineering and science has not found much value in high cost cables, alas, but very high quality pre-amps for reasonable prices are quite easily obtainable.

Your system is only as good as your weakest link.  This doesn’t mean you have to spend gobs of money.  It means you spend on quality gear that gives you your desired sound.