The vast majority of hi fi speakers available today are made for powerful transistor amplifiers, which are often very power hungry and can be a difficult load to drive for any amplifier, especially valves. This is especially true of smaller stand mount and bookshelf speakers where huge amounts of midrange and treble are literally “dumped” in order to increase the relative bass output.
Obviously better results will be obtained from you valve amplifier if it is driving an easier more stable load. For example many loudspeaker designs use a four ohm bass unit, working on the premise that the amplifier power will double going from eight to four ohms. This of course does not happen with valve amplifiers; therefore the sound balance will be wrong. A well designed efficient loudspeaker with a more constant impedance will give vastly improved performance in terms of volume level, special image and more solid controlled bass.
A simple yet effective crossover ensures that the signal transfer from the amplifier has the minimum impact in order that the amplifier can “see” the speaker drive units clearly without excessive distortion.