Non-feedback amplification topology and frequency

Is it true that non-feedback topology amplification is prone to frequency limitations if not matched with the absolute perfect speaker system?
I'm trying grasp how to get a flat sound from my very much loved new PL5. I have resorted to EQ, that never mentioned term on these forum, to bring back the ultra-highs of cymbals and bells etc. I've also rolled the pre-tubes and switched out my ICs. Still very much a frown freq graph.
I'm willing to get new speakers to help this amp deliver on its promises. Focal seem nicely high in sensitivity.