I'll throw in another possibility for 'extra' tubes in a preamp that can improve fidelity, cathode followers after again tube. They supply a nicer impedance match to the next gain stage. If every gain stage in a preamp used them you end up doubling the number of tubes.
Another possibility is paralleling tubes to lower an amplification stage output impedance(like a cathode follower above). I haven't seen it used that much but I recall Conrad Johnson doing it on some of their top of the line products.