The idea with most loudspeakers (but by no means all) is to drive them with something called a 'voltage source'. A voltage source simply is able to make the same voltage regardless of load.
Tubes can do this, but only up to their power limit, which in this case is 150 watts or 300 watts depending on the amp above. To get a better match, an output transformer is needed. To maintain the optimal load on the tubes, the transformer has taps for 4,8 and 16 ohm speakers. So regardless of the speaker impedance, the most you will get out of these amps is 150 watts for the 150 watt amp or 300 watts with the bigger one.
Now here's something to keep in mind. It does not matter what kind of amplifier you have, it will make more distortion into a lower impedance (like 4 ohms). This distortion is audible as increased harshness and loss of detail. So your amplifier investment dollar is best served by a higher impedance speaker if high end audio reproduction is your goal. If sound pressure is your goal you have a 3 dB reason to go with 4 ohms, but only if you have a transistor amp that can drive 4 ohms with double the power it can do into 8 ohms.
In addition, speaker cables are far more critical when used with 4 ohm speakers.
In a nutshell, its not a good idea to make any amp work hard- the harder it has to work, the more distortion (harshness) its going to make.
Tubes can do this, but only up to their power limit, which in this case is 150 watts or 300 watts depending on the amp above. To get a better match, an output transformer is needed. To maintain the optimal load on the tubes, the transformer has taps for 4,8 and 16 ohm speakers. So regardless of the speaker impedance, the most you will get out of these amps is 150 watts for the 150 watt amp or 300 watts with the bigger one.
Now here's something to keep in mind. It does not matter what kind of amplifier you have, it will make more distortion into a lower impedance (like 4 ohms). This distortion is audible as increased harshness and loss of detail. So your amplifier investment dollar is best served by a higher impedance speaker if high end audio reproduction is your goal. If sound pressure is your goal you have a 3 dB reason to go with 4 ohms, but only if you have a transistor amp that can drive 4 ohms with double the power it can do into 8 ohms.
In addition, speaker cables are far more critical when used with 4 ohm speakers.
In a nutshell, its not a good idea to make any amp work hard- the harder it has to work, the more distortion (harshness) its going to make.