Leben 300 rather than 600 IMO.
I've used a Jadis Orchestra, a Jadis DA30, QUAD 33/303 and Roksan Caspian integrated with my P3ESRs and P3ES2s (and M30s and LS3/As).
I've never heard any Naim kit so cannot compare, but in my system I prefer the Harbeths with valves for the top and mid-range sweetness and imigery, though the Caspian (now sold) drove them very well, with plenty of bass grip. The QUAD is more valve-like to my ears.
Although not a Naim fan myself, I'm slightly surprised at the OP's comments, as Harbeths seem very popular with Naim (and Croft) etc owners.
I have only heard them with the 300 and not the 600 and they sounded very sweet. Not to say they don't sound as good with the more powerful amp.
I used P3 ESRs with an Exposure Super XV. I would not say the were smooth or lush, they were ruthlessly accurate and positively sparkled on bright programme material. It remains the best small room amp/speaker combo I have ever used.It's worth mentioning that the OP was using the Naim NAC172 to drive the P3ESR. Perhaps that was the reason the OP found the combination to sound dull, lacklustre and lacking in detail. The internal Burr Brown DAC and preamp section of the 172 are likely the culprit to the poor performance. Apart from the quality of preamp, the source (DAC) is also very important as the Harbeths are inherently a soft and smooth sounding speaker.
FWIW I tried the Nait XS + decent source with the Harbeth SHL5 and it sounded decent, though not spectacular. I believe the Nait XS is a better amp than the 172 due to the higher quality preamp section. Combined with a good source the Nait XS can easily surpass the 172. The 202/200 brought more detail and life to the Harbeth, and the 282/250 gave even more.