Without knowing your denon specifically you will likely gain very little to nothing.
While I'm more that satisfied with using my 818's internal amps, the $800 I spent mostly went to Audyseey / THX / HDMI ports.... licensing and connectivity. You generally pre-amp to quality power or don't bother.
Unless you just want a few more channels powered, at which case your gonna run an entire Denon avr just to get 2 extra channels, going from the Denon to the Onkyo makes even less sense as you loose the 818's processing.... which is why you bought it.
Thanx for info. Thing goes like this I have 5.2 system with big front AR9, surr Mission MX6, cen Kef R600, 2x active subwoofers mission & jamo. When I go over 75 volume sound get so harsh and distorted its pissing me off. Its like amp can process some hard music HM :)
No I dont whant to have 818 as preamp for 3802 (its opposite) I will first kill my self than do that.