This is a relatively quick fix if you don't mind parting with the cash needed.
The Rodman....ad infinitum, pegged it. Expose and exrtirpate them immediately. It is the the classic British hallmark- warm- richly colored That warmth and euphonic middbass boost makes them very desirable and the bestr choice So, just don't rid yourself of these Mullards too quickly.
The alternatives are too numerous to give a good accounting of. Nevertheless by rolling just the 12AU7s you will in all likelyhood find the sound you like. The Germans are clear as a bell. taught firm quick big sounding tubes. They are no doubt a great choice if you have inherited a vast fortune. Or like me just thought a few Telef..ECC802S lying around would be a good idea.
If you don't feel overly flush you can pick up all varieties of USA tube brands such as an RCA chrome getter, top not clear IMHO or the in terms of playing with various types like the 5814 by CBS etc.
You will know when you put the right one in very quickly.
My personal favorite is unfortunately expensive, but are clean resolving but more detailed that are not marching in lockstep with the Germans... These goe by the number 7316. All of the 7316 are Amperex Heerlen Holland tubes. By definition but labled many things Mazda, Mullard!, etc. Look for the CTL Delta ### codes. That is by far my favorite 12AU7. I like the longer plates, halo getter versions best They are usualy from around 1960. You won't be listening to a vastly different tube no matter which you choose. The 7316s spanned mid 50s to about 1970, long or short grey plate D or Halo getter. As a group they are fairly expensive however people manage to spend a great deal more on other tweaks that do nothing good to my ears. Best of all they last, so if you like them you got the and if you don't they hold value. I have been using a set for over 3 years on and off. The sound is highly resolved images well and very slightly mellow.
Still I would and did try the cheaper stuff first but avoid the thin scratch the blackboard cheap Chinese like the plague. The old Soviet block is a mixed bag but always sound "off" as if it was not a 12AU7 at all but something stronger and starker just not the a 12AU7 sonics.