It's a solid state amp if I recall, so you can't really hurt it (without trying). If you put an 8 ohm speaker into it and it was designed for 4 ohms, you're going to lose a lot of level.
But speakers are a complicated beast, and there are other factors at play here. The most notable is speaker efficiency, which is usually rated in dB per watt input. In other words, jam per watt.
If the 16 ohm speaker has a sensitivity rating of 97 dB/watt, and the 8 ohm speaker has a rating of 103 dB/watt, this means that the 8 ohm speaker transmits more energy to the eardrums per watt of input (usually stated at a specific frequency).
And then there are all the factors that go into how good a speaker sounds for the type of amp and guitar, but we won't get into that.