From a previous poster on this thread to which I agree. - "Lots of factors here.
Sound hole size, wood, bracing, build quality, action, strings and on and on."
I'm rolling with myth though, and here's why. I'm 72 years old, when I was 18 years old I bought a 1968 Gibson B-25 that had the 1-9/16" nut. I loved the thing and played it daily until it was stolen in 2000. I bought a Gibson Hummingbird Montana division model in the mid 1990s just because I'd always wanted one, I liked it a lot, but I liked the b-25 more, they both had a very similar tone, but the b-25 was easier to play and as I recall had very little if any difference in loudness. I sold the Hummingbird in 2010 to buy a 2010 Martin D-18 Standard, because I liked the Martin better, but I still missed my b-25. There's something unique about the b-25s, the narrow-necked ones made them much more playable, and their tone is beautiful to my ear.
I came across a mint 1971 cherry sunburst b-25 deluxe with a 1-9/16" nut on Craigslist several weeks ago. I live near Columbus, Ohio and the owner lives in Cleveland. I was ready to pay the $1,850.00 he was asking which included a like-new original case and a very nice hardshell case, or do an even-up swap for my d-18. He drove down and we played and discussed the guitars for close to two hours, and he took the swap. We were both happy with the deal. The string height of the b-25 was so high I pretty much knew it would need a neck reset, and it did. I should have it back from a top-notch luthier who lives just outside of Mount Vernon, Ohio on Monday. A neck reset and a compensated bone saddle will cost me $550, but I still feel lucky to have found my old sweetheart.
Here are two YT videos both made by the same excellent guitar player, one is a 2016 Gibson Hummingbird "Tone Test" and the other is a 1965 B-25 "Tone Test" which didn't have the X-bracing of the 1971 deluxe model. You be the judge of which sounds the loudest and the sweetest, I already have.
Hummingbird-
https://www.youtube.com/watch?v=Sbs44KiuGwI
B-25-
https://www.youtube.com/watch?v=d2SiOb3nI2Y