I think the default setting on a TSL is 16 Ohms anyway, so just plug and play (but check first!).
Changing the impedance setting is pretty easy if you need to though.
Don't forget though, that if you add another cab, get another 16 Ohm cab, and then set your amp's output impedance at 8 Ohms (2x 16 Ohm loads create an 8 Ohm load). You can't mix and match output impedance loads, say using a 16 Ohm cab and an 8 Ohm cab, without causing damage to your amp.
