It's your pre-tube sounding smoother and less fatiguing. Everything should match on all channels. Would you put a SS amp on your right channel and a tube on the left? Most of us know the importance of matching the L & R speakers on 2 channel system. Some go to extremes measuring speaker angles, speaker placement, exact speaker wire lengths etc... Home theater is just the same but you are adding more channels. The most seamless presentation will have identical speakers, amps and cables for all channels. Unfortunately there is not much to choose from for HT if you want to run a tube pre/processor. Products like this would make pretty good sense since SS amps are generally better for dynamics which are prevalent to cinema. The tube front-end would smooth things out as you have already discovered. Consider also that movies are non-stop sound for 2-3 hours. Anther option would be to just get rid of the center and enjoy your tubes in phantom mode.