I haven't seriously auditioned receivers of late but some general observations on sound characteristics of Denon vs. Yamaha vs. Onkyo.
On HT, hayop yung processing ng Yamaha. My bet is the DACs used by Yamaha are a cut above the rest. I'm not saying that Denon and Onkyo sucks but the surround processing on the Yammie is sweet.
On stereo, Onkyo and Denon edge the Yamaha a tad (soundstage breadth and depth). Bedyo flat mag-image ang Yamaha which is susprising since they have good DACs. My guess is it's the amp section that's causing this.
Ang amp section ng Yamaha at Denon medyo lean at cool tumunog. Be careful when matching with speakers with metal-dome tweeters (as many budget speakers are). Baka maging masyadong bright. Mission, B&W, Tannoy comes to mind as good batches for brighter sounding receivers.
Personally, among the 3 you are considering, I'd spring for an Onkyo (on sound character alone. note, I deliberately choose not to use the term "on sound QUALITY" as I expect that I'd rank these receivers differently depending upon the system it'll be used in). Onkyo is warm sounding which makes it a good match for a wider range of speakers (PSB, Acoustic Energy come to mind). Note though, I am a NAD receiver and a Nakamichi stereo amp user so I am biased to warmer, richer, fuller sounding gear.
Good luck and have fun choosing. Tell us what you decide.