I think you mean what you asking is about studio monitors. It's important to step back and take the entire environment in to account though when talking about mastering. I use a Dangerous Music Monitor ST to handle routing from outputs of DACs to various sets of monitors I'm using. This device allows me to gain up or attenuate inputs as well as monitor outputs. This stuff is critical in mastering because you want to be able to reference the mix at the same volume as the master so you can ensure you are actually making an improvement.
For monitors I use Focal SM9s with a Dynaudio Sub, a pair of Avantone MixCubes, and a small Bose radio. All these are connected to the Monitor ST and I can call them at the push of a button, turn the sub on and off, mute the left and right channels etc. The Bose is a strange addition but they tend to exaggerate the bass and highs and they have no EQ capabilities. This means it's a consistent signal every time. I don't use it all the time but every now and then I'll flip over to it just to hear if the bass is a bit too much or maybe a crazy high hat is jumping out too much or something.
The final piece is the room. The room was acoustically designed and properly treated. All of these together make up the listening environment and all play their part. We mix here too which is one reason I didn't opt for high end more hi-fi speakers as monitors.