there are many studios that do mixing, and some studios that do mastering; very few that ace both - and millions of opinions if you ask what is and isn't good. there are also your pair of ears that you should trust
if you plan on learning some self-mastering, i'd suggest you learn how to use "peak" audio editor (
www.bias-inc.com) and buy vintagewarmer & masterq plugins from pspaudioware.com