As tubes age they at the 'end of their lives' sound pretty lackluster.The gold lion tubes that PS Audio recommends are fine tubes but emphasize the mids and bass.A faster sounding lively tube is the JAN Phillips 12AU7.It's one to put on your list to try.
PS Audio BHK lacking..
I bought a used PS Audio BHK preamp to replace my Rotel. It sounds thin with no depth & bass compared to my Rotel so I let it run for over an hour with no change. I'm new to tubes so I'm looking for ideas. New tubes or what?