Check out p. 29-34 from the Mesa boogie manual, which has a great explanation of impedence matching, and also says which mismatches are safe, including configs with multiple cabs, etc. In particular note, from p. 29
"You can always have a higher resistance (16 ohms, for example)
without damaging results, but too low of a resistance will likely cause problems.
MIS-MATCHING: When running a higher resistance (for example: 8 ohm output into 16 ohm cabinet), a slightly different feel and
response will be eminent. A slight mismatch can provide a darker smoother tone with a little less output and attack. This response is
a result of the amplifier running a bit cooler"
http://www.mesaboogie.com/media/User Manuals/3chRecto.pdf