The answer your first question if you can replace the fan on that Hyper Evo, YES you can. I will also quote: "Utilize the included brackets to quickly add or remove fans for upgrades, replacement, or maintenance." Just make sure you buy the right size fan, as you can get anything from 80mm - 200mm. That fan on the Heatsink is 120mm, so make sure that is the size fan you would buy.
Secondly, reducing DB really depends on the fan you get. The Db numbers you get from the listing page ( newegg-amazon-ebay ) are for the particular fan it comes with. If you buy a different fan, if the DB numbers are lower on the new fan then it will be quieter.
Now personally, I wouldn't concentrate on LED's. When I got my water cooler, it came with just a stock corsair fan, I switched that out for a Cougar Vortex HDB, which is a really great fan. Hope this helps