I owned M-5Si years ago. They do like power. I used an Aragon 4004 on them and they paired very well. That amp is long in the tooth. What kind of budget are you talking about. If you could swing something like a Parasound A-21, it should work well.
One of the demos that really caught my attention years ago was a pair of M-1Si vertically biamped with a pair of Krell KSA-250s. That demo was what convinced me to buy the M-5Si.