Well, for the most part, the stock sound system that the Rogue Sport come with is really "bassy" and there not much you can do with it.
You can always buy some Pioneer/Kenwood aftermarket speakers and have them replaced. This is not a big deal at all.
That should brighten up the sound.
The stock speaker location on the "S" is on the lower door panels. Not good for sound. BUT, the upgraded sound system which has tweeters located on the dash bouncing off the windshield is very bad as well. Not much you can do except upgrade the stock "Whizzer" cone speakers. That will improve the sound. I'd look at a highly efficient speaker and perhaps one that the tweeter could be rotated and aimed towards the driver. I'd leave the stock head unit alone since you're leasing. Last pair I bought were for my Ford Focus were Infinity's for the rear door panel. (I had previously installed some old stock Boston Acoustics for the front locations a few yrs prior.) Driven by the stock head unit. Excellent sounding and I'm an audiophile. A quick check on Crutchfield's shows a 6 1/2" speaker for both front/rear. Best for can do w/o spending big $$$ and then you're not guaranteed great sound unless you go component and relocate the tweeters to a better acoustic location. (NOT on the front dash just under the windshield.) Hope this helps. Let us know if you change anything and how much of an improvement it makes. Bill.