How to change the volume of one pair of speakers
From a quick search, the Wharfedale E20 have 95 dB at 1W, 1m sensitivity. That's quite high. The Kef on the other hand have lower sensitivity, 85dB at 1W, 1m. So the latter will produce 10x less acoustic power with the same input power (watts). So there is no actual problem, just different speaker efficiencies.
If you add a resistor in series with the more efficient loudspeaker, you're making a voltage divider with R2 being the loudspeaker and R1 being the resistor.

Thus, voltage on the loudspeaker is given by the usual voltage divider formula:
\$ V_{out} = V_{in} \frac{R2}{R1 R2} \$
Problem: here's an impedance graph for a random &quot;4 ohm&quot; speaker from the internet...

As you can see, speaker impedance varies substantially with frequency, and thus the divider ratio will do the same. So the tone balance of the loudspeaker will be affected, and it will sound different. You can still try it though, for example a 68 ohm resistor of suitable wattage, like 5-10 watts.
Another option would be to get another amplifier and use an amp per pair of loudspeakers, so you get two independent volume knobs.