I had your same pre but with McIntosh M601's and later MC75 III. Until you experiment with better NOS tubes, you are missing a great deal. As a dedicated tube addict, I ran every NOS 12AX7 (and 5751) and 12AT7 out there....my favorites by a wide margin were the Sylvania JAN / JHS Triple Mica Blackplates and for 12AT7's I also preferred the Sylvania 12AT7WA. Try that combo and see what you think. The differences won't be subtle.
Enjoy!