Sam Byrams wrote:

What are the more popular mastering systems now? Who ( just a few
names) is using what?

Just about anything you can imagine. There are a number of market segments
for studio speakers. General monitoring, which can also be broken down into
narrower segments like tracking, mixing and mastering.

Each application. can be conceived of as needing a different type of
speaker. In some cases, there is a controversy over whether any of these
applications must be implemented using speakers, or whether headphones or
IEMs might be appropriate.

Non-loudspeaker monitoring approaches are particularly popular for tracking
and mixing. Mastering now arguably may include non-loudspeaker approaches,
since we now have ten of millions of listeners using portable players. Thay
are using headphones and increasingly, IEMs. Computer speakers are another
popular listening environment that may need to be considered.

A major trend over the past 30 years has been the ascendancy of small
monitor speakers, sometimes called "Near field" or "meter bridge" speakers.
Small studio monitors have been a trend going back as least as far back as
the BBC's LS3/5A. Over the years the bass extension and dynamic range of
small monitors has evolved and improved greatly.

Mastering itself can arguably be something that isn't best done with just
one set of speakers or a single listening environment. Instead, some (myself
included) tote recordings they produce around to different listening
environments, take some notes, and go back and make adjustments as it seems
appropriate.

During mastering, the major issues are dynamic range, balance between direct
and reverberant sound, imaging, and tonal balance. Hopefully these aspects
of the recording have been at least roughed-in during mixing. Tracking is
more about the quality of individual playing. There seems to be no end to
the possibilities for fine tuning at any step in production.

Very few recordings are targeted towards just a single narrow playback
environment so they should at least to be QCd in a number of different
sonic contexts.

I strongly agree with other poster's comments relating to the sonic
impoverishment of many once-widely respected legacy speaker systems. OTOH, a
few other legacy systems don't do badly when compared to modern systems,
particularly with a little adjustment, some careful eq, an added subwoofer,
etc.