I would suggest by rolling in the 5751 and see what happens. Be weary of the cathode follower positions, as I recall reading some posts that the Russian tubes do not last very long in those circuits. Not sure on the JJ tubes since they are from a different manufacturer as the more common tubes made in Saratov (Reflektor company that makes Sovtek, Mullard, Tung Sol, etc....) I am uncertain of the cascade on your particular amp so it may have certain effect where it is placed. If you get one that has matched triodes, it may even work well in the PI position (usually results in more headroom). You could also use a 12AT7 ( which is about as low a gain you should go). There may be some alternatives in the 12AX7 family that do not have as high a gain as the JJ since there are many to choose from. Each different type of tube will also effect tone, some are bright where as others are darker. For example, Mullard CV4004 seems to have a scooped tone to it, where as the Mullard reissue is quite the opposite. Keep in mind, it depends on the topography of the amplifier circuit on each triode, since I do not have your particular amp, it is hard to be precise as to what will work well for you.
