In my opinion YAMAHA NS10's are not the best speakers for mixing and/or mastering audio. I "grew up" on them, I should know. However, you may be me surprised at my next statement.
There's really no "BEST" speakers for audio mastering or mixing.
Here's why.
I've been an audio engineer for over 20 years and I've worked in many different recording studios with a variety of reference studio monitors aka speakers. I've also worked with some of the top names in my genre of music as mixing and mastering engineers, and I have came to the conclusion that Speakers used for mixing and mastering audio gain their popularity and credibility in various ways. 1) Great marketing and branding, 2) When a hit record is made using said speakers, 3) When a person of "authority" uses and speaks publicly about said studio monitor. There are many more reasons but these three I believe to be at the top.
Yamaha NS10's were not the only popular mixing and mastering studio monitors.
Have you ever heard of the KRK ?
KRK's have been around for as long as I've been an audio engineer. This picture shows the KRK Rokit 5 & Rokit 8, they also make the Rokit 7. Many audio engineers and recording studios believed them to be the best in their class of speakers. These bad bays had a bit more low end than the NS10s, that's probably why they were a staple in most reggae and hip hop studios.
Yamaha discontinued the NS10s in 2001. Realizing there was still a market for them, they then released the self powered HS50M, and after a few updates its now the HS series. there's the HS5, HS7,HS8. These sound nothing like the NS10s to me, remember I "grew up" on NS10's. I worked with them for more than 10 years, I still love them. However, they come pretty close.(HS5 pic below)
Avantone, another giant in the pro audio space, most popular for their version of the classic cube style reference monitors made by Auratone (Original Avantone5C brown, Avantone mixcube black), Also makes a version of the NS10's. its called Avantone CLA 10, branded by award winning Producer/Engineer Chris Lord Alge.
I haven't personally used the CLA10 but there are reviews on youtube you can check out.
Let me know what you think.
Bottom line is there's no BEST mixing and mastering speakers simply because it all boils down to a matter of preference. The room you are working in, and who introduced you to whatever monitors you are using also plays a role in this. Its kinda like fast food. Some people like McDonald's, some Burger King and some Wendy's. It is as simple as that. So my advice to you if you're in the market for mixing and mastering speakers.., go to your nearest pro audio store and listen something you produced, mixed or mastered on a few different monitors, compare them, then make your choice based on if you like what you hear coming from the speakers.