Read your manual....
http://mesaboogie.com/manuals/Studio%2022%20Plus.pdf
"SPEAKER JACKS One 8-ohm and two 4-ohm speaker outputs are provided on the Studio .22 Plus. Use the 8-ohm jack when using only the internal combo speaker. Also use the 8-ohm jack for connecting an external 8-ohm cabinet (such as a Boogie 4x12 cabinet) without the internal speaker. If using an external 8-ohm cabinet and the internal speaker, plug them both into the 4-ohm jacks. If using an external 4-ohm cabinet (such as a Boogie 2x12), connect it to the 4-ohm jack (and
preferably disconnect the internal speaker). You'll be nicely surprised at how well your Studio .22 responds when driving bigger cabinets... incredibly loud for its rated power output!"