If they play as loud as you want, without any audible distortion, when being driven by your Onkyo, then you won't gain anything from a separate amp.
However, if they begin to sound strained or distorted before you get the volume as loud as you'd like, then it might be worthwhile to try an outboard amp.
Gear: Speakers (Paradigm Reference Studio Series): 40 v4 (front L/R), CC-690 v4 (centre), 20 v4 (surround L/R). PSA XS30 (subwoofer).
Denon AVR-X4000, Emotiva XPA-5, Samsung PN64F8500 (64" plasma), DarbeeVision Darblet DVP5000, PS3, Sony BDP-S6200, Apple TV (3rd gen.), Harmony One, Sennheiser RS180