I can't give you a conclusive answer, but I can raise some issues. I have a temporary setup in my living room happily running on an older MacBook Pro and a USB 2 hub, but I'm running a FireWire audio interface, so that could make a difference. Here are some things to keep in mind:
Is your MacBook Pro pretty much brand new? AFAIK, only the very latest models support USB 3. [I just checked - the mid-2012 MBPs were the first to support USB 3.] The Axe-FX II itself does not; it's a USB 2 device. It will still work in a USB 3 port, but not at USB 3 speeds.
Most hubs, either USB 3 or 2, will run at the speed of the slowest device you've got plugged into them. So unless the hub specifically states that it is a Multi-TT hub, plugging in the Axe-FX II could force everything connected to that hub to run at USB 2 speeds. If your dongle is old, it could even be USB 1.1... even slower! A serious bottleneck.
You should always use AC powered hubs... but if your USB 2 device slows down a USB 3 hub, it could also reduce the amount of power available for USB 3 devices like a hard drive, and they might not work properly or at all.
I realize this might not be much help, but AFAIK, the only MBPs with USB 3 also have Thunderbolt. [After checking, this is indeed true.] In that case, definitely get a Thunderbolt external drive; your Axe-FX II will be fine plugged into the USB 3 connection, but your dongle could still slow it down if it's not USB 2. Good luck!