"And another question I have is are 15's known to have a muddy sound?" YES!!!
However, before we sound the death alarm, on single 15's, it truly does come down to the enclosure.
Best advice: 4X10, minimum. Happy sound. Happy Life.
Not all 15's are the same and calling all 15's muddy is a generalization that a lot of folks will disagree with, including me. Depends on the 15. The Eden 115 and used to be 215 cabs come to mind as mudless cabs. The DNS-115 by DNA is anything but muddy. And, I sure wouldn't call the Mesa Subway 15's muddy either.
I toured with a nightclub group for 3-years in the early 70's starting with a 212 cab on my Fender Bassman and within 2 months I traded it in on a Standel piggy back amp with a powered bass reflex cab with 4 round ports mounted vertically next to the 15's. Yes the two 15's in the Standel sounded different than the two 12's in the Fender. They weren't as tightly focused as modern speakers are but they were far from muddy. Played about 750 performances with that group before we broke up.
The newer 15's still had that rounder sound than you get with 10's or 12's but they are not like the stuff we had in the 60's and 70's. And
@Auspuff is spot on that it depends on the enclosure and how they are matched to it.
One bit of a warning though on 410 cabs. I have a great one and when I bought it in 2015 I though that was the last cab I'd ever have to buy. And then I started rolling it in at some small venues in the Seattle surrounds (brewpubs mostly) and the owners about had apoplexy. "Oh no", they'd say, "that's way to much for here. Use that 112 combo I have for you over in the corner". And of course they always had crap combos. I ended up buying an Eden EX-112 to use with one of my amps at small venues and it sounded so good I bought a second one. Then I'd use two of them at medium size venues, saving my 410 for large indoor venues, outdoor venues, and music festivals. So I already bought two more cabs after my 410.
Then I was accepted to the DNA Endorsing Artist group and bought a DNS-210 to use for both small and medium venues because no one looks twice at a bassist bringing in a 210. It's a particularly loud cab with 700-watts power handling, and ended up being my go-to cab for all indoor venues. So now my 410 is just used at outdoor venues and Music Festivals.
If I had it to do over, I would have bought the 210 first because most of the places we play are small and medium-sized venues. Then I would have bought a second 210 for outdoors and music festivals. I feed it with a DNA-800 amp which will put out up to 400-watts in my 8Ω cab. If I had a second one, I could add that and then have an 800-watt 410 when needed. As it is now, my 410 stays in the closet most of the time.
Another thing about good 210 cabinets. They often (not always) have the lowest and tightest bottom end of all the cabs from a manufacturer. My DNS-210 will play the fundamentals down to 34Hz. It's a full and very nicely focused bottom end. So the old saying of, "you need 15's to get the best bottom end" hasn't been true for decades now. They do have a different character to their sound, but often the 210's will deliver lower frequencies better. Having a modular set-up where you can take one cab most places and only pull out the second cab to make your rig into an 800-watt 410 when needed, is a really nice way to go.