First off, your M9 should have come with Line 6 PX-2 power supply, not a One Spot. Even it was used when you bought it, the people at GC should know better. I think I'd go back and ask them to give you the correct supply.
There are many threads about the M9 and DC power. I won't say it won't work, because there are people who have been using theirs with DC power supplies for a while. Two things - First, I suspect that 300mA will not work for very long. The M9 draws something more like 1100mA when all effects are on. Second, none of Line 6's testing for the M9 has been done with a DC supply. Just because it works for a while doesn't mean there aren't components that could potentially be damaged in the long run. Because of this, I don't see a good reason to use a DC supply. I know it's convenient and all, but in the long run convenience may end up costing you more.