I think it's the same everywhere. You get good staff, and bad staff. Those who are happy to help, and those who are looking for a fight.
However you also get good customers and bad customers.
My tactic would usually be:
a) Start off as friendly and polite as possible, explaining the problem, why it matters to you (watch a lot of dark movies or whatever), and ask if anything can be done. We all know about consumer rights etc, but remaining polite at this stage (and keeping such threats out of it) often gets things resolved fairly quickly as it puts the staff at their ease. Even smile, charm, and drop in small talk - it all helps to do this. Going in guns blazing is a 50/50 job, because then you're triggering their "fight or flight" instinct which means you'll get either a never-ending argument with the employee standing his ground until security come, or an instant refund, respectively. Alternatively, the "flight" could be towards a supervisor, starting the 50/50 process all over again. Too much of a risk IMO.
b) If that fails, remaining friendly and polite (geniune, not sarcastic), re-state exactly the same thing. "Well, the thing is I really do watch a lot of dark movies and I'm really not sure this screen is up to par. I've seen others which don't have this problem. Are you sure there's nothing you can do to help?".
c) Prove it - get them to put theirs and yours side by side with the same video (keep a copy on CD), or borrow a friend's good one, and test it in an environment where the problem will show up. Then re-state the case again.
d) If this still fails, ask - nicely if possible - for the supervisor/manager (better hope you have proof at this point, as supervisors tend to go more "by the book"). Start roughly from "a", being kind and polite etc.
e) If he too is awkward, now's the time to start mentioning consumer rights, that a product should be fit for the purpose for which it was advertised etc etc.
f) Still no luck? Ask for the complaints procedure. Put in a polite but firm complaint, sit back and wait.
g) If you can be bothered, now's the time to start threatening then going ahead with court action. Sure you could suck up the 10% for the sake of a peaceful life, but that's what the retailer is relying on... depends on the price of your principles. (Admittedly, I'd probably give up and suck up the 10% at this point and refuse to deal with them again, but I doubt it'd come to this).
NEVER mention that you heard about the problem on an internet forum. That's just asking for trouble. The internet is generally not seen as a reliable source, especially forums, so that'll usually earn an immediate dismissal of the problem EVEN IF they've admitted it elsewhere. Also you could be seen as only raising an issue because someone else told you there's one etc. As far as they're concerned you should have discovered the problem yourself, and possibly have a friend with the same issue or a more reputable printed news article or something which you discovered afterwards. It's "I happened to notice this problem", not "I heard about this problem on the internet and I think mine might be affected"
Good luck to those returning! Hoping I get a decent one first time
