I would guess that it's the room. Hardwood/tiled floors? That's the biggest culprit. Bare walls/mirrors? Also, anything between the speakers can cause early reflections (uncovered bay window, equipment and racks, TV etc). Are the speakers toed-in? Try toeing the speakers out (straight ahead). Also, CD's are particularly nasty sounding if the sources are not quite so good. Silver cables? That too. The correct impedance tap on the tube amp? Many many things...but try the room first. I would use a sound meter to see where the problem is and consult an acoustics specialist (Rives Audio, eg) for tips or solutions.