The leading medical expert on this topic in the US is probably Beth Darnall https://profiles.stanford.edu/beth-darnall She uses Binaural frequencies around the 7-8Hz range for pain management but they only work with headphone because the the low frequency comes from the dissonance between two notes slightly apart playing to each ear. iTunes has a whole Binaural playlist for Alpha, Beta, Theta brainwave frequencies if anyone is curious about the relaxation side of these frequencies.
Schumann Resonator
I got 2 of these from Amazon...careful that free returns are applicable. I charged them up, turned them on and holy moly.....they do help with my system. What I hear is clarity....space between instruments, a definite difference in upright and electric bass, wider soundstage...you know...all the good stuff. At first I thought it might be increased brightness, but no....it is still the same in that regard. I still can't believe it, and will listen again tomorrow (saved the packaging for the return)...but today, I'm about to keep them.