Another thing to keep in mind is impedance - the M200 is rated at 200W for an 8ohm load, but it seems to me that many subs present something closer to a 4ohm load. The M200 can easily work with such loads, and in fact can offer 300W of power when faced with a 4ohm load.
The only difference I can tell is the balanced input and a slightly higher output rating on the A1.
Actually, the
A1 is rated the same as the M200 - both are rated at 200W for 8ohm loads, although NHT lists the A1's output as 250W (which is assuming a 6ohm load).
I look forward to hearing how things work out for you, mojonator.