It's always best to make sure that the impedence matches between the amp and speaker. This is very crutial with tube amps, but usually isn't much of a problem with solid state amps, such as the MG line. The main thing you need to be sure of, is that the speaker impedence isn't less than the amps. So, an 8ohm speaker should work fine if your amp runs at 4ohms.

Use the search function of the forum to do a search on this topic. It has been discussed numerous times.