Who's using Harbeth with tubes, what speaker model and how many watts per channel?

I use the 8 ohm taps with any Harbeth that I have used. I never heard any difference between the two taps! This is from Stereophile!
Harbeth P3ESR loudspeaker Measurements

Sidebar 3: Measurements

I used DRA Labs’ MLSSA system and a calibrated DPA 4006 microphone to measure the Harbeth’s frequency response in the farfield, and an Earthworks QTC-40 for the nearfield and spatially averaged room responses. Harbeth specifies the P3ESR as having a sensitivity of 83.5dB/W/m. My estimate was slightly less than this, at 83dB(B)/2.83V/m, but this is within experimental error of the specification. This is a significantly lower sensitivity than the norm, meaning that the little Harbeth will need a fairly powerful amplifier to play at acceptable levels in all but small rooms. However, its limited power handling (50W program) places a strict upper limit on the loudness it will produce. The Harbeth is claimed to be "easy to drive," and indeed, its plot of impedance magnitude and phase revealed that to be the case (fig.1). It is also an easier amplifier load than its predecessor, the HL-P3ES2. Other than a dip to 5.7 ohms at the bottom of the midrange, the P3ESR’s impedance remains above 8 ohms for almost the entire audioband, which will make it a good choice for use with tube amplifiers, provided they can swing enough volts to drive it to high enough levels. However, the shape of the impedance curve suggests that the balance will tilt up a little at the high end with such an amplifier.

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

I have the 30.1's and drive them with an Audio Research GS-150 tube amp (155 watts/channel).  Also use a AR GSPre (tube pre-amp).

I’m appreciating everyone’s posts. Thank you, so much great information.

Sorry to chime in, because I don’t have direct experience with tubes and Harbeth...Only my Croft (hybrid) amp, which is 45 watts per channel. That has driven my (now sold) P3’s, and current C7’s. I have a fair size space...23 X 12 X 8 ft ceilings. The consensus is they like power, yet quite a few Harbeth owners are running low power. A friend runs a 22 watt SET with his 40’s, and loves it. I have not heard his setup. If I had the scratch, I’d love to try a mid/high power KT 88 or KT 150 amp. I’m in budget mode right now, so I’m thinking of the Belles Aria amp. My hope is more power will make the C7’s come alive. I’d also like to hear them with a Luxman 550 class A amp.