@OP GHD Prentice's post sums up the amplifiers very well. I would also add that if you like your Harbeths, then you will like the Pass. However, if you predominantly listen to rock music or large scale classical, and depending on your room size and seating position, you might want something with a bit more power - but that needs to be real power and not just watts on paper.
Will an Amp change improve my system ?
My system is WiiM pro ( a great device for $149 !) and a Cambridge CXC V2 CD transport for digital input going into a Schiit Yggdrasil+ DAC going to a Schiit Freya+ preamp into a PS Audio S300 amp into Harbeth 30.2 XD speakers. I am considering replacing the S300 with one of the following: PASS XA-25, Coda S5.5, Benchmark AHB2, or tube Primaluna EVO 400, used Rogers EHF-100 or 200, used Luxman MQ 88uc. I like my current system and wonder if an amp change would improve it significantly ?