Is there a big difference between them? Do you have to run a pre am with the M80's? What would be the reasons to go up to the M80's?

You can see that the 80's have one more tweeter and 1 more mid than the 60. The 80's are rated at 4ohms, which can cause problems with some receivers/amplifiers. I don't have either speaker but the consensus is that the M80 will give you more bass and a larger soundstage. If your current system can drive 4ohm speakers you won't have to purchase anything different to run the M80. Now, there are many receivers/amplifiers that aren't rated for less than 8ohms but have no problem doing so. Denon and H-K are 2 of them.

Various Axiom folks have said that the biggest difference between the M60 and M80 is that the 80s can handle more power (presumably because of the extra drivers) and play more loudly. They also go a few more Hz into the bass because of the larger enclosure.

Some users have reported greater difficulty positioning the M80's in their room, presumably because they are, well, bigger. Both speakers are fairly deep dimensionally, and unless you have a very big room or play your content quite loudly, you will likely be quite satisfied with the M60.