Signia and Rexton have three music programs to choose from; Listening to live music, Listening to recorded music, and Playing an instrument. They are mostly reducing noise reduction and feedback suppression. Feedback suppression seems to be an issue that can distort or cause pure tones to warble. So part of the solution in addition to turning down or turning suppression off can be to use a more closed fitting to reduce feedback potential.
There have been a number of threads here discussing this issue of music and whether or not digital aids are better or worse than analog. You may want to do a bit of a search.
My recollection is that there are a couple of specifications to pay attention too. One is the input dynamic range of the hearing aid. This is not a common spec and you have to dig for it. I recall that Widex was one of the first to claim increased dynamic range, and they are currently at 112 dB. Signia claim 113 dB, so essentially the same. Another, possibly ReSound Quattro claims 116 dB?
Another is the frequency bandwidth. Most are now 10 kHz, with Signia claiming 12 kHz for the 7Nx. For many of us, this is a moot point and we are not going to hear anything that high. The other thing to watch is that when the receiver power goes up, the bandwidth goes down, and the distortion goes up.
Hope that helps some,