Terry,
Speakers in most of time don't have stable impedance. The tube-friendly ones usually specked to have minimum of 8Ohms or larger. Tougher speakers can go down to 2 Ohms that you don't want to use for tube amp. The ouptut impedance of amp should either match or be lower than speaker impedance so if you have for instance 6Ohm specked speaker you should connect it to the 4-Ohm tap to keep your tubes safer than on 8Ohm taps.
Theoretically you can refer to above para, but practiclly especially if you don't listen loud, you can connect to 8Ohm tap if it sounds better to you. If you like to listen loud I would recommend you to refer to para above.
Speakers in most of time don't have stable impedance. The tube-friendly ones usually specked to have minimum of 8Ohms or larger. Tougher speakers can go down to 2 Ohms that you don't want to use for tube amp. The ouptut impedance of amp should either match or be lower than speaker impedance so if you have for instance 6Ohm specked speaker you should connect it to the 4-Ohm tap to keep your tubes safer than on 8Ohm taps.
Theoretically you can refer to above para, but practiclly especially if you don't listen loud, you can connect to 8Ohm tap if it sounds better to you. If you like to listen loud I would recommend you to refer to para above.