IMO if you want to use a sub for music 2 channel, you should place the sub in a center channel position(between the mains). Its the only way to get it to blend IMO. Sounds fantastic between the mains and can even be set at a high cross over setting
IME, I would agree with this. Dead centre between the speakers produced the best result. A caveat, the sub needs to be reasonably high quality. If the sub is poor quality, you will get poor results wherever you plonk it. I learned through the hard way.