I agree with Alex.
Out of personal curiosity I downloaded the Splawn manual - not very helpful as it gives no clue how the two speaker outputs are wired. They could be in parallel (most likely) in series or even be wired to switch the transformer tap when both sockets are used.
I have an old Fender concert and that automatically selects a different output transformer tap (same idea as the manual selector) when you plug a second speaker in - but both speakers/cabs have to be 8 Ohm - I think it puts the sockest in parallel so it will select the 4 Ohm transformer tap.
On most Marshall amps the two speaker outputs are wired in parallel so if you use two separate 16 Ohm cabs (or speakers in your case) then you need to set the output selector to 8 Ohms. Far easier and cheaper to wire them in parallel in the cab and then you know exactly where you are, as Alex said.
Sorry to repeat what others have said but using the amp with the wrong impedence selected wil not do the output transformer any good and the same applies even if you ditch the Splawn.
Pete