This is probably one of the hottest debated topics ever. The only way you will know is to try for yourself. Personally, my system has benefited from better cables and the difference was noticeable. Comparing cables is something I've done quite a bit and there is differences. Some subtle some pretty obvious. I guarantee you put some Transparent cables between an amp like the Parasound and Dynaudio speakers you will hear a difference. You say you are budget minded yet have put together a good system so if you want to get the most out of it you should at least try some better cables. Most any hi fi shop worth their salt will lend you some to try.

That phsycological nonsense you've read irritates me to no end. Didn't you hear the difference in your amps or speakers when you evaluated them? Then, why would cables be any different? Besides that you don't buy them up front, you either borrow them or buy under a 30 day return. I'm not shooting the messenger, I've heard that crap before. That is an insult to you, me, or anyone who takes this hobby seriously. If we aren't competent enough to hear differences then why do we have the gear we do? Just because you can't explain why it happens doesn't mean it doesn't happen. It wasn't until recent years that anyone claimed to know how a bee flies but it didn't keep all of them on the ground.

I'd be interested to know what you meant buy "custom" on your main speakers. Dynaudio builds all their speakers by hand including the drivers and coils. It would have cost a fortune to have custom drivers built.
At some point your system would benefit from a matching, or close to matching, center channel speaker. To my ears there's a pretty good difference between the presentation of Dyn's and B&W.