Using a tube amp for the mains will not make a timbre difference, it will only make a listenability difference - the mains will sound better. A good tube amp should have a flat frequency response, which is the main determinant of timbre differences. The center really can use a huge dollop of tube amplification because this channel carries the dialogue which more often than not sounds harsh to begin with. There will not be any huge timbre mismatch though if it is solid state.