Interesting that the LFD LE V doesn’t have the current to optimally drive the P3ESR. When I used the LFD LE III to drive the Harbeth SHL5, the sound is very open and dynamic. The combination sounds a lot better than most of the amps I have tried (with the Harbeth).
Having said that, you may be right about the low sensitivity of the P3ESR. At 83.5 dB it is surely on the low side compared to the SHL5 and C7ES3 which are rated at 86 dB. Also, the SHL5’s impedance is 8 ohms while the P3ESR is at 6 ohms.
The Harbeth surely sounds different with different amps. A powerful amp is certainly useful if one listens at higher volume levels (especially the larger speakers SHL5 and 40.1 / 40.2). A friend who owns the 40.1 has used many amps on the speakers (he currently owns all these amps) - LFD LE III, Exposure 2010S, Leben CS600, Naim 32.5/135 monoblocks, and the Naims got the vote. As good as the Leben and LFD sounds, they lacked power, and they lacked some neutrality of the Naims. The LFD made everything sounds a bit larger than life while the Naims gave a more accurate portrayal. The LFD’s strength or appeal is its tube-like glow which gave music a nice illuminating feel.
You might want to give Exposure 3010S2-D a look.