Tubes most likely to improve SQ?

I have a newly purchased integrated tube amplifier.  The tubes consist of 2 x 12AU7, 2 x 12AX7, and 4 x EL34.  All of the stock tubes are chinese tubes labelled by the amp manufacturer..  They may not be, but look very much like Psvane 12AU7-T without the gold pins.

I have owned many excellent SS amps (Krell KSA-100, Parasound A23, Parasound A21, Classe AMP2 and a few others.). I bought this amp on a gamble and am surprised how amazingly good it sounds into my upgraded Tekton Electrons.   I have tried GE 5 Star 5814 in the place of the 12AU7 and was very impressed. Now I am considering a bit more tube rolling.

 Am I likely to get much change/improvement from changing the 12AX7 or the EL34?