Here's my take on this, based on years of experience with only Onkyo HT products...
First, I own the 605, which is the spiritual model grandfather of the new 608. Aside from the 90 to 100 watts per channel power jump and THX certification and other improvements, the guts are similar as are features I am going to get to...
If the OP's max volume is topping off at "91" instead of "99," then somewhere in the calibration chain, some channels (or the "IntelliVolume" input trim control) are being set into the "+dB" range -- let me explain. When any channel calibration setting in the AVR is set to a "+dB" number, the AVR compensates by lowering the overall master volume by that amount. The same thing goes for "IntelliVolume"...so, you won't have "99" as the max volume number any longer based on those settings.
In my system, all my channels are in the "+dB" range because I prefer the channels "hot" like that, and I am running Onkyo's IntelliVolume input gain trim for my "DVD" input (where my Blu-ray player is connected) at "+10dB"...this is yielding a greatly reduced maximum volume value, but I am getting very loud audio at a lower number on the display readout -- and hence what I think the OP is looking for.
Under the SOURCE SETUP of the 608's SETUP MENU, look for "IntelliVolume" for whatever input you have connected which you feel is "low in output;" if you bring this number up above "0dB" the audio will sound much more forceful and powerful without needing to get to that "Max" readout on the 608's display. This is something I was actually going to begin a thread about, and probably will after this issue of the OP's is resolved; that is, using Onkyo's IntelliVolume system as kind of a power amp's "input level gain" control.
At any rate, the point here is that if any speaker level is brought into the "+dB" range on Onkyo AVRs in addition to IntelliVolume being run in the "+dB" range, the overall maximum volume number will be reduced according to the summed number from all those adjustments -- this is why I think the OP's max volume dropped to "91". That said, I cannot imagine getting to max output with my 605 with standard action films or music; perhaps MAYBE if I am playing a REALLY quiet source like the audio tracks on the original Star Trek Blu-rays (which are recorded RIDICULOUSLY low) but something is definitely off if he can get it to full output without distortion or blowing his ears out -- setting IntelliVolume to a higher number will allow the amp to seem "more powerful" without needing to crank it to max...