The MG-I is rated at 82dB/w/m at 5 ohms input. However, the truly magnaplanar design as these are offer up a nearly resistor flat impedance curve so the 5 ohms is easier to drive tham equivalent sensitivity 8ohm speakers. Depending upon the size of your room, you can get by satisfactorily with that 50wpc amp of yours. I used a cheap 45wpc amp for a few months and found the combination very enjoyable. I've run everything from a 15wpc 6BQ5 Fisher SA-16 amp up to ~200wpc solidstate and found them to sound good regardless. However, this is in an ~15'x16' room (technically larger thanks to an L-shaped design).

If you room is a great deal larger then definitely look for a high-current amplifier more than the watts. Maggies respond more to current than other speakers and you will appreciate the results.

Good luck.
- JP