Midrange Increasingly Harsh

I replaced the Mullard  EL-34 tubes with Tung-Sol EL-34B tubes and after some run-in time I replay the Hildegard Von Bingren CD which started this.  The sound is much improved and not nearly as harsh.  The higher register soars rather than screeches.  I don't have back-up 12AX7 or 12AU7 tubes so need to get a set of them as well.  I'll roll them in to see how things change.

Congrats!

Is impossible to get good sound from low grade speakers, you can do anything but it does not help. 

FWIW, I've noticed some distortion in my PL amp. I changed out  the power tubes - different tone but distortion still there. Short version, I finally just put in new drivers because I had back up ones of the same type/brand, etc, because I like their sound. Bingo! Everything's fine now a the cost of just two 12AX7's. 

@jrcotner 

This is an extremely common problem call sibilance. The problem is that our ears are most sensitive at frequencies in the 3 kHz to 4 kHz region exactly the same frequencies reflections in most rooms are loudest. It is more common with dynamic speakers with broader radiation characteristics. Horns and planar dipoles are not so affected because they are directive.  The solutions are use an EQ curve with a Gundry dip in it. Or get down with some room treatment. You need to put absorption at all first reflection points walls ceilings and floors. Or you could get yourself a set of horn or planar speakers but you might still require some room treatment although it should be a lot less. I Use 8 foot tall ESLs 36" wide. The only room treatment I needed was three rows of 4" foam acoustic tile directly behind the speakers. Still the occasional recording will cause some sibilance due to the way they were mixed.