The problem with using pre-outs is that you would probably not have a convenient means of disabling the sound from the speakers while listening to headphones.
It looks like your AX-7e has tape outs, which presumably are "ahead" of the volume control in the signal path and therefore should be suitable, as was suggested above (assuming, as is likely, that the headphone amp has a high enough input impedance to be suitable for use with the tape outs).
Not sure what "tube buffer in my epi" refers to, or how that might be utilized for headphone listening.
Regards,
-- Al