with respect to which tubes, perhaps i should not have made my prior statement with such ’absolute-ism’ - it obviously depends on your amp and your room
most harbeths are nominally 6 ohms, dropping to just above 4-5 ohms in the bass region
my understanding is that tube amps typically have trouble delivering a lot of current (certainly compared to ss amps which happily double their delivered power into 4 ohms vs their standard 8 ohm load specs), thus the care needed in making a tube amp drive loads that are low impedance in nature... furthermore, low impedance is most common and hardest for the tube amp to deal with when it is in the bass region, where cone/motor motion requires the most energy (this current, given a specified voltage level) to produce the strong bass notes
the tube amp in turn uses its output transformers to ’leverage’ the voltage differentials developed by the tube circuit into current flow, thus the lower impedance taps provide more transformer windings aiding the amp do this when called for...
obviously, at the end of the day, the circuit theory and governing concepts need to work in practice... your amps may do just fine driving the little p3’s with its 8 ohm taps... it is worth trying both if your amp has both (my audio research ref 75 amp has 4, 8, 16 ohm taps for instance) - as they say, it is a ’free at home trial’
in my own experience with arc and primaluna tube gear i feel there is little better drive and bass control to my c7's or super 5's at the 4 ohm tap, but a little more sparkle and spotlit nature up top and through the mids at the 8 ohm tap - difference is slight in magnitude, but i believe i do hear it consistently
