If that's the case, I don't think a 200 W amp would solve your problem. The Sierra 2-EX seems to be the limitation because it's sensitivity is anechoic sensitivity is only 85 dB/2.83V/1 meter and the power handling specs are 150 W maximum, 300 W short term peak, grille off. so if you want to listen to them at reference level you are going to have to sit very close to them.
Audiovision.de tested the RZ820 that is very similar to the RZ830, probably the same preamp/power amp sections.
audiovision.de
The reviewer claimed "............In terms of power measurement, the TX-RZ830 offered a few watts less than its predecessor, which, however, does not really matter with the still lavish reserves of power at all loads: with 7 fully utilized channels, it was still very good 100 (4 ohms) or 94 (6 ohms) watts are available and increases to a proud 231 watts in stereo mode (4 ohms). . "
So what is your seating distance, and when you said "if I really crank the volume...." what was the volume setting?
The Onkyo specs say -81.5 to +18 on the relative scale. Did you go pass "0"?