Do expensive cables/wires REALLY make a difference

Not all cables reproduce music through my speakers exactly the same. I have had some that cost more than others and the differences were slight to extreme.

After a long journey over the last 35 years I've settled on a complete system of MasterBuilt interconnects, speaker cables, and powercords. Now, anytime I replace any part of the set, even one pair of interconnects, it's easy to hear how the system is compromized. I have the Reference series and they are expensive. There's a no hassle return option. You might want to try the Standard line first and see what you think. If you try them, give them several weeks to break in.

Your dealer is right in that a power conditioner can make a system sound worse, but it's also true that a conditioner can make a dramatic improvement. Which one it is for you depends on how clean/dirty your power is, what components you use, etc. Find a way to try one or two conditioners from the better companies (i.e. Audience, Shunyata, BPC, etc.) and then you'll know. I think most (but certainly not all) people find they make improvements and have some form of condioning in their system, and then there is the added protection from spikes/surges.

I don't think someone can say difinitively whether or not to plug your amp into a conditioner. It depends on several variables, and once you have a conditioner in your system it's free to try it both ways. If plugging into the wall I'd suggest at some point having a dedicated line installed, which you should do anyway (it usually costs less than $500, which in this silly hobby is one of the cheapest upgrades you can make). And yes, a conditioner still makes a difference if you replace the stock power cords. When it comes down to it, in my experience almost everything makes a difference. Whether it's an improvement or not is another story.

Depends on the cable Stealth V10 speaker cables do.