Also have a look at Fabric for Unity 5. It comes with a lot of good features to help implement your audio well, and is native to unity.
You have access to mix states, mix bus and also DSP as well as any other DSP that Unity provides.
Unity native audio showcases some of the DSP functions with unity 5 - the side chain compression is something you may need to use (also a feature in wwise).
Fabric Unity Audio Engine provides a lot more than the basic unity audio - tempo based music switching..etc though I don't know just yet how complex a music system it has.
Both Fabric and WWise can be obtained free to experiment with before choosing one to license.
Dynamic mixing for games is definitely an art in itself and a lot of fun to make it work
It really depends on how technically your music is adaptive. Does the mix just change because of one stem gets added - ie there's a basic mix and then another stem is layered in, or does the music need to get to a transition point and change. That will drive what middleware or solution you choose as well. I have not explored Fabric's dynamic music system at all, but have used WWises and FMOD desgner's a lot. Basic knowledge of FMOD from talks and demos I've had with the guys and the previous designer system both FMOD / WWISE have outstanding dynamic music systems and authoring tools where you can prototype the behaviors.
Both also work with unity, with an external plugin. It's so great to have so many choices these days!
As far as standards, a lot of developers for console / tv are adapting the ITU 1770 standard of loudness. WWise actually supports metering with this to help achieve the right mix. For handheld / tablet.. etc - there is no real standard yet since we're dealing with different speaker types sizes.