Alright... So after an hour of troubleshooting, and a ton of Google searches, I managed to figure out that Noctua's provided screws simply don't play well with the HAF XB EVO. Given that installation using rubber plugs would require a complete teardown of the system, I reckon I'll install the back fan once I get a new CPU cooler.
As for the 200-mil up top, it's quieter at full blast than my stock Wraith Prism cooler is at idle. Good stuff, well worth the ~$30 I paid for it.
Word of advice: Don't buy Noctua fans for a silent setup. They might technically be whisper-quiet, but the high-pitched fan whine of both my 120mm and 200mm models drove me insane till I figured out the cause. Instead I think I'll try out something from be quiet! once I get my next paycheck.
Honestly, I think Noctua fans are kind of ugly anyway, the default color.
Function > form!
I meaan... There comes a point when you oughta concern yourself with form. For me, that's when your case fans harmonize perfectly with a restaurant chocolate milkshake. ^_^'