^^
I don't think anyone can definitely advise anyone on which particular AVR is going to sound more "dynamic" or "less bright" or "warmer" than any other brand. I almost replied earlier but Mantis raised the point

If "dynamics" are what you're after, what do you think the Onkyo is going to do better than your Pioneer? The amps in the Pioneer have been tested to not drop too much power 5 & 7 channels driven simultaneously. I haven't found any bench tests on the 3009 but why would you expect it to out-perform the amps for ability to reproduce peak power demands?
Unless you 1) don't like the sound of the 55 or 2) want Audyssey Multi32XT, IMHO, all you're accomplishing is spending money to trade laterally

Most Brand x vs Brand y posters are just seeking people to agree with a decision they'd like to make anyway

My advice is go listen for yourself & compare. If this "really good deal" on the Onkyo is an internet deal, then see if they have a 30 day return policy. If it's a storefront dealer, many will let you do demos in their store or in home but it still won't be the same speakers, unless you're lucky or the same acoustic environment.
Setup some listening comparisons in
your home with
your speakers. We don't know the speakers you have, their impedance or the size of room you use. How can anyone here tell you one would be better than the other for "dynamics"?
The most important factor to your sound is the
Speakers
Room acoustics
The electronics behind them make a very small (or miniscule) contribution. As long as the amps are capable of driving your speakers & you're not driving them into clipping, changing amps won't change "dynamics" much if at all, unless they have a lot more peak power capability. If anything the Pioneer should "win" since it's already to come close to its power rating - 7 ch driven at same time....what more do you want in a mid-priced receiver?
If it's a lot more headroom you seek, then add a hi-powered ext amp, in the 200-400wpc range, like a Parasound A51, for example.
Changing to get a different room correction EQ system is a different story. Are you unhappy with Pioneer MCACC? Do you prefer Audyssey or want it to do sub EQ? If the answer is yes, then by all means get the Onkyo or go back to a Denon.
Nothing wrong with swapping gear, but if you've already swapped brands 3 times in a short time, maybe the issue is not the gear but you don't know what you want.
Just because it's a deal doesn't make it better

and deals come along all the time, so why be in such a hurry?

Instead of doing a knee-jerk reaction to chase a deal, find an Onkyo dealer and go listen! To me, it looks like you are a price shopper, chasing deals instead of taking time to investigate, talk to your dealers, audition & listen for yourself.
Sorry I'm direct but it's in the interest of helping you.
Edited by ss9001 - 8/5/12 at 7:51am