As Dave said it draws 60 watts, so the heat it dissipates will be about the same as the warmth from a 60w light bulb. If there is no venting to let heat out the unit will probably get a bit warm over time while it's powered up. There's likely one spot where the power supply is located that may be warmer than the rest of the unit, maybe the back right corner where the AC cord is connected(?) I'm guessing not too many here have one of those, so getting an opinion if it runs hot might be iffy. If still unsure perhaps an email to Mackie might get an answer.
On the bright side, the heat won't let dampness build up inside on the components
.