Don't change the parallel-feed output cap (3.3uF), it interacts too strongly with the tuning of the speaker it's driving.
You can change the interstage cap (0.1uF) to give a first-order rolloff. But the driver stage will still have to handle the full range, and might overload on strong bass transients.
Or you can add a cap in series with the input to the amp to protect both tubes from the deep bass.
Or you could do both the interstage and input caps, giving a second order high-pass function (with a Q no higher then 0.5).
