String quartet or saxophone quartet?

There really is no instrument that is any “better” than another for assessing truthfulness of timbre (within a given frequency range). It depends on just how intimately familiar one is with the sound of a particular instrument; and personal preference for that particular sound. Every instrument or combination of instruments has its own unique timbral characteristics which pose unique challenges to the process of record/reproduce.

I prefer jazz but I will say I find strings to sound amazing on my system.

Don’t worry. Be happy. Blast both of them (well, maybe the string quartet could be set to a natural sound level). But to me, full orchestral and opera (preferably Blumlein-miked) are the ultimate tests.

“Daniel Deffayet Quartet”, probably the greatest saxophone quartet ever:

https://youtu.be/3Zu6uXNX0PI

https://youtu.be/5WYUawPukaE

“Prism”, probably the most prominent quartet today:

https://youtu.be/7SsstJC4mEk

https://youtu.be/z7gQ3fSOaIw

String quartet are a good test for an audio system...Timbre textural density of all instruments playing together in their own space is one of the test by which you know if your system is good or not...

My sentiments exactly.  

String quartet are a good test for an audio system...Timbre textural density of all instruments playing together in their own space is one of the test by which you know if your system is good or not...

I am not fond of sax, i prefer a quartet of trumpets, but is there only one?

All jazz are almost sax filled.... Less trumpet playing.... sax is more sensual, but trumpet is more ethereal....

String quartets, just because there is so much more repertoire. Not Mozart, nor Haydn, nor Beethoven, nor Brahms, nor Shostakovich wrote any saxophone quartets.