Amp for Harbeth P3ESR and Cambridge Audio Azur 851N

I have gone with option 1. Which is Naim uniti Atom + Harbeth P3ESR. I felt the sound is a little thin to my liking. It lacks lushness, power and details. I want to add amp between the uniti Atom and Harbeth.

Any recommendation?

BTW, I moved the setup to my bedroom. The sound does improved due to smaller enclosed place compared to living room. This is expected. But still something is lacking....

My P3ESRs sounded amazing with my ARC VSi55 integrated.  It ran pretty hot in the small room I had it in and I recently switched to a Vincent SV 237 integrated.  It incorporates an amp with the first 10 watts in class A with a tubed preamp and has maybe 95% of the magic of the ARC, but in spite of the Class A runs much cooler.  I'm very happy with it.  I have no experience with the other amps you mentioned, so can't compare.

What about adding Vincent SV-500? SV-237 is quite high price above $4.5k. Not prepare to spent that on this set up. Just not sure if SV-500 will help in my setup.

You might be thinking of the newest Vincent SV 237MK, however it retails for about $2,400 USD (are you in another country?), not $4.5K.  The unit I bought was a second hand Vincent SV 237 (no "MK"), the previous generation.  I paid $1250, or $250 more than the SV 500.