Hello Tomscy,
Hope this finds you (and your canine friend) well. I have a set of Heir Audio 4Ai. This was my first set of IEMs, and I have no experience with other brands. My main reference (other than live acoustical music) is my main system. It is flat in frequency response, except the bottom octave is about two dB hot. This system produces a very realistic soundstage and amazing depth of image. I find that my HA 4Ai, RS Shadow, CLAS, iPod classic, Toxic cables set up is quite similar in sound. From your description, I feel that I might like the Music 100 over the Pro Audio 100. I really like my universal IEMs, so I want to get a set of customs. These look like a possible alternative to Heir Audio and their somewhat higher price. I realize that this may be comparing apples to oranges (single vs multi drivers). Curious to know what you thoughts may be. Thanks in advance for your help, and thanks for taking time to write such a great review.