Mixing Tube and SolidState Amps

Has anyone heard of using a tube amp for one spectrum of sound and using a solid state amp for the other? It seems to me a friend of mine once used his McIntosh tube for the ?high end? and a tube amp of some kind for the ?lows?.

I shipped a Onkyo 8070 to a girlfriend of mine today. she has a tube amp. she also has speakers that have 4 poles on them. 2 for high, tow for low.

any help would be appreciated.