External Speaker for Marshall DSL15c

[LesterPTelestrat]

I'm thinking about adding an 8 ohm 10" speaker (a cab I just happened to have laying around) to one of the spare 8ohm outputs in the amp and leaving the 16ohm internal speaker connected. This gives me a combined ohm value of 5.33 ohms. Marshal says not to exceed or use less than 8 ohms. What could possibly happen? premature tube life? Fried transformer?

Any thoughts? is it worth the risk to my amp to try this?

Thanks for any assistance ya'll can share.