Speakers that disappear

@willy-t wrote, "pick music that doesn't have widely panned instruments and or vocals"  Oh man you hit a nerve there for me!  This is one of my biggest peeves.  Sound engineers often try and do too much and overwide panning has ruined too many songs that we all love.  I find that when engineers try and spread the mix as wide as possible the sound gets trapped and never releases from the speakers and just makes mud.  

For the OP to realize his goal when auditioning new speakers I would recommend that he hand picks some tracks that are very well recorded and that are known to produce a good sound stage.  I would look for music recorded by Alan Parsons and Ken Caillat, they are two of the very best at getting good sound that allows the speakers to disappear.

One other thing to keep in mind when auditioning is that more complicated music with more band pieces is harder for the speakers to render.  Simple acoustic or small ensembles can be successfully reproduced by any entry level Best Buy speakers but the bigger more complicated stuff will separate the wheat from the chaff during your demos.  Good luck and cheers.

check out the monitor audio platinum 100s if it's in your budget or the gold 100s.

@willy-t  has it right.  Ignore all the specific speaker recommendations.

Quality of Recording is really important, placement of listening chair.  And volume.

Unfortunately, speakers are always at the mercy of recording quality.

While I agree that all speakers “can” disappear to some extent or another and that things like the room and recording quality also matter significantly, some speakers still just inherently and more easily and completely disappear as a sound source better than others regardless of other factors.  Joseph Audio does this and I’ll add ProAc in there too as standouts although there are certainly some others that pull this off well.  So to say individual speaker recommendations are not helpful in this regard is simply not accurate in my experience because certain brands do excel at disappearing as a sound source better than others, and that’s what the OP was asking for here.