I've tried my M4 into the return of my old rackmount SLO. It was ok, but definitely wasn't anything to write home about. I later picked up a VHT 2/50/2, which was much better, but I still was not happy with the low-end. The top-end was nice & clear, but the low-end seemed to be shelved off compared to the SLO. Shortly after that I tried a friends 2:100, and his 20/20. Both were ok, but again, nothing to write home about. The power amp I settled on, and have stayed with, is the RT2/50. It sounds excellent, is relatively inexpensive, and has the ability to switch between 6L6 and EL34, which is awesome if you try to recreate tones like I do.
Just remember, your rigs ability to 'punch' is not just from the power amp. You need to consider string gauge, pickups, preamp saturation, EQ, compression, power tube choice, the cones, and even the wood of your cabinet.