I think the most pronounced differences you will hear will be in midrange/upper-midrange tonality and, if you have a well set up system, the imaging, which in pentode should be more forward and pronounced in the mid-range. Triode (in those amps I have which allow selection) usually is more recessed in the mid-range, and has a recessed soundstage and (in my stuff) more narrow as well. FWIW.
