As mentioned, the Monolith is made by ATI which is one of the best sources fo home audio amplification.
However, the Outlaw 2200 is made at the same facility that makes Parasound amps! Although I have more respect for ATI, Parasound is certainly no slacker.
Power amps are a very mature science and you should not expect either of these companies to have a significant (audible) design/performance advantage.
Obviously, the monoblocks will have less crosstalk between the channels, but the level of crosstalk for the ATI/Monolith is simply not an issue!
In practice, these options are essentially equivalent.
Both are sweetheart products because they are well designed/built units being sold without the level of markup normally associated with a brick & mortar network of dealers.
If I were in your shoes, I would look to other considerations such as cost and appearance (and fitting in your console as well as 3 vs 1 power cord) to make the call.
It wouldn't surprise me one bit if your Onkyo is capable of managing all 5 channels. How loud do you listen? Do you hear distortion from your speakers? Which speakers (generally you might assume the amp is clipping, however, some speakers distort when pushed)?
Depending on your speakers and subwoofers, it may be wiser to spend the money on something that will make a more obvious improvement in your sound quality than adding amplification!
If you have your "final" speakers, and have the coin warming your pocket, go for it and buy amplification!
