Aesthetix Calypso: Bugle Boys vs Mullard 10M

I have not had the 10m's but when I had my Calypso I rolled many tubes thru it. It responds well to tube rolling and you are able to customize the sound to a great degree. 
I preferred 5751's over 12ax7's. Slightly lower output but with the dip switches set to high, it really wasn't noticeable. 
The two tubes you mentioned are quite different. BB is a cleaner, faster tube while a Mullard is a warmer more romantic tube with a softer top end.
My preferred combo was Raytheon Windmill getter 5751's and Mullard 2492's.
There are many good choices and like anything in this hobby, they are system dependent.
My suggestion to you is to buy several pairs and have fun. You can always resell the ones that don't make the grade.

Give Andy a call at Vintage Tube services. Tell him you have a calypso and what you expect out of it. He will suggest tubes that meet your needs.vintagetubeservices.comCached

The Mullard 10m tube's are at a level of performance the bugle boys cannot match, clarity, speed, tone,etc..., the 10m are not as warm sounding as the other Mullard tubes, if you get authentic 10m mullard's, we'll then,  congratulations,  these are very expensive and rare! , the performance after break in of the 10m is legendary.