Originally Posted by stevo
Maybe I have this wrong, but adding an 8 ohm speaker to a 4 ohm system should be fine although you might lose something in tone or volume. It's the other way that would be problematic - you wouldn't want to wire something for lower reactance than it's rated for since that would actually increase the current that it sees. So if the amp is actually rated for 4 ohms, it doesn't get much more robust than that.
The general consensus over on the Weber amp building forum is that in a tube amp you can run between 1/2 or 2X the amp's rated impedance with no risk of damage, but there may be a slight impact on tone/performance. It's SS amps where you don't want to go below the amp's rated impedance. Actually in tube amps running below the rated impedance is somewhat less risky than running over the rated impedance (which is why some tube amp speaker outputs use switched jacks to short when a speaker is accidentally not plugged in. There's less risk of damage running a tube amp on a dead short than an open circuit).
Weber has great 10" speakers available in 4 ohms.