Does it matter what speaker impedance I use?

No – as long as it matches the output impedance of the amplifier that it’s plugged into. In a valve amplifier the output transformer’s windings match the speaker to the power valves. So the amp will sound the same if you use a 4-ohm speaker plugged into the 4-ohm output, or a 8-ohm speaker cabinet plugged into the 8-ohm output, etc.

There are a few postings around the Internet that claim it’s always best to use all of an output transformer’s secondary winding, which usually translates into using the 16-ohm output with a 16-ohm speaker. Assuming the output transformer has been correctly built, there’s no reason why there would be a significant difference. And no matter how many times I’ve experimented with this, I can find no tonal difference.

For myself, I tend to choose 16-ohm speakers purely for consistency, and it makes it easy to mix speakers within a larger cabinet if all of them are the same impedance.