I have earned my money as a mastering engineer for the last 15 years. Quite early I started to build my own gear. A few years ago, I changed over to the 500 series format, because I have always appreciated the modular concept of the old Neumann mastering desks.
Over the years, my system has grown up and expanded. While it is still not complete, it is sufficiently developed to offer it for sale as a Mastering system.
The main advantages of a modulare system are, you can choose the most interesting modules to complete your existing system for mastering and/or even for use on your mix-bus or as a group processor.
Every now and then comes the question why there is still no compressor. Over the last 10 years I spend most of the time with EQ and filter design. Three years ago I started with different compressor designes to employ. But that will still take quite a while until I have presentable results, if at all :-)