I use a couple ADR Propak Audiomates as a M/S matrix - one for encoding and one for decoding - they were a good bit cheaper than the Dangerous stuff. I had them modded though - replaced the stock JRC opamps with National Semiconductor LME's (which are very low noise / low distortion). I also had the input pots taken out of the signal path.

Transformers are very cool - and I definitely love their sound as I have them in a number of processors - but if you're trying for "transparent" transformers aren't necessarily the best thing to put in the signal path. fwiw - I've found that a relatively inexpensive opamp based solution for a M/S matrix can work very well.

http://www.adrl.co.uk/pdf/AudioMate%2520Datasheet.pdf

Other possible alternatives are some of the older Neumann or Telefunken "panorama" x/y / M/S modules which are transformer based (although from what I've read the headroom on these isn't all that great). Another possible alternative is one of the discontinued AEA M/S matrix's - Audio Engineering Associates – Products – M/S Stereo Legacy - although this would probably be hard to find.

Best regards,
Steve Berson