My own personal take on the instrument timbre issue is that the speakers are
critical (given that the rest of the system is well matched good gear) but
not for the usual hifi reasons. I think what really helps to achieve that
extra 'magic' is NOT having wide dispersion speakers as it bounces the
mid/treble around the room too much with the ears receiving multiple
reflections. This lessens the sense of hearing an individual instrument
working in it's own acoustic space as was present at the original recording.
It becomes a blurred mixture of the original acoustic and the listening
room's acoustic signature. Conversely, old speakers like the Quad 57 tend
to beam the sound straight at the listener resulting in a hypercritical
stereo sweet spot, but you can hear the original acoustic because of that
flaw and it can help make an instrument sound more 'real'.