Talks about "opens up" is just another hearsay BS, unless it is quantified/defined, just my 0.01C.
I doubt most receivers in the $1K price range would have trouble with slew rate to the point it becomes easily audible. There is no such thing as square music wave anyway, and all periodic waveforms are the combined waveform of harmonics. Fourier had it figured out more than a hundred years ago. As long the total harmonic distortion is low enough I wouldn't worry about the slew rate spec too much.