Thanks for your valuable comments. The studio I use has invested in a ribbon mike (Beyer M160) - just after I finished a session using harmon mute !. for my recording they used a high quality dynamic mike a Sennheiser MD421. I have also since heard that a ribbon mike is probably not a good choice for live work because it is mechanically fragile and can be damaged by phantom power through a faulty mike lead. They also found a useful thread on the gearslutz forum
Help with Harmon mute - Gearslutz.com
I'm going back into the studio just to try various combinations of setup, mike, positioning and eq just to get the optimal sound characteristics - if anything interesting comes of this I'll post it.