What is meant when someone says a component is lacking soul

It means it is not going to heaven. (Or something like that.)

Recently had a glaring example of this when I sent my amp away for extensive modifications/rebuild and pulled out my slightly modded Adcom 555 back-up amp.  Gave it a couple weeks to come to life but never happened. What I wasn't hearing was any note decay, any hall sound.  The notes played but that was it.  Dynamically, it was like going from a Bosendorfer to a garden variety harpsichord.  No ebb and flow to the music!  No "life".  Or you could call it a lack of "soul".

To paraphrase Issac Hayes: Fresh hot popcorn without salt and butter.

 In my circle I might say of great sounding and feeling music playback "Man, that's filthy!"

IMO, "lacking soul" means flat or blurred dynamics possibly accompanied by alack of transparency and / or harmonic detail.

True musical detail is most often a matter of electronics that possess very accurate timing.