According to the LSS manual, the "optional" output is matched when running an 8 ohm load at the 30 watt setting, but you can also use the 15 and 5 watt settings with an impedence mismatch.
Similarly, the "main" outputs are matched to an 8 ohm load for the 5 and 15 watt setting, and the 30 watt setting produces an acceptable mismatch.
The LSS doesn't have the output impedence labeled, but given that you are using 4, 2, or 1 tubes for the 30, 15, and 5 watt settings respectively, I think that the output impedence changes depending on the power setting as follows:
30 watt--> optional output = 8 ohms, main outs = 4 ohms
15 watt--> optional output = 16 ohms, main outs = 8 ohms
5 watt--> optional output = 16 ohms, main outs = 8 ohms
I have tried all configurations in a variety of settings and prefer the main outs when running in any power mode. I usually have ch 1 set to 30 watts, and ch 2 set to 15 watts and I'm plugged into an 8 ohm speaker. At 30 watts, it's looking for 4 ohms, but an 8 ohm load is an acceptable mismatch.
I have used the optional output with an 8 ohm speaker, and it seems a bit tighter on the 30 watt setting, but there's little difference on the 15 watt setting.
When set to 5 watts, the sound of the amp doesn't seem to change at all when plugging the speaker into either the optional or main outputs. This may be because the 5 watt setting distorts so quickly.
Unless you use only one power setting, you will have a mismatch on one or the other settings. However, all mismatches using an 8 ohm cab should be acceptable.