DJ,
Ironically, I recently (about a month ago) installed a Denon X2800 myself to replace an old Onkyo TX-SR605 that we loved (it started to fail on the HDMI OUT port, which was important for me so I could check the onscreen menus from time to time), and I have had nothing but issues in terms of functionality and loudness differences.
First of all, I would definitely use (even though it was suggested not to upthread by one member; he's not really wrong, per se, but let me explain why I'd recommend it) the Denon's Source Level adjust feature; in my old Onkyo, there was something similar called "IntelliVolume," and this balanced out the preamp levels for each source connected to the amp. However, many of us used it instead as a sort of input boosting stage, much like old power amps of the past that had the input sensitivity control; so, by setting, say, the DVD input to "+10dB" on this IntelliVolume scale, the overall sound from that input would be much louder at a lower master volume number. It's just what I preferred, even though it's never recommended. The newer Onkyos dropped this feature, which is one of the reasons I did not go with Onkyo for a new receiver (the other major one being that they also stopped the name of the sound mode and input to be displayed at the same time on the front panel, which was definitely a dealbreaker for me).
The Denons do have this Source Input leveling feature, and I use it for the "Blu-ray" input we use on the new X2800; I leave this where I left my old Onkyo's IntelliVolume setting, at "+10dB," and while this has helped, this new AVR STILL doesn't sound as punchy, aggressive or in-your-face as my Onkyo did. Now, in touching on what lovinthehd said above, it is indeed difficult to compare numbers to a previous amp -- but I totally understand the temptation and desire to do so, because I do it as well. Every night we use the system.
We're set up on the ABSOLUTE volume scale, not RELATIVE, and I calibrate all my AVRs myself without Audyssey or any other kind of room correction/EQ. When I set up the Denon, I plugged in the exact same speaker trims, crossovers and distances as I was using with the Onkyo, and this, in theory, should have been fine because nothing about our seating environment changed. Using the exact same calibration values, I am finding I have to turn the Denon far beyond -- arbitrarily speaking -- what I kept the Onkyo's volume at to achieve anything resembling a similar output.
For example: on the Onkyo's ABSOLUTE scale (the only one available on that particular model), the system was screaming loud (depending on the way a disc was mastered) at 37 to around 40, perhaps even lower. With the new Denon, the "40" mark seems to be where the soundtracks just begin to pick up steam and appear loud-ish, and because of the way the Denon's remote stages the volume increases and decreases via "0.5" decibel steps, it sounds like nothing is getting louder or lower unless you REALLY go to extremes with the button pressings.
I am finding that I truly miss either the Onkyo "house sound" or the way their units' amps were set up in terms of bias and gain; I understand your pain in all of this...to the point that I am considering boxing up the Denon and just taking a financial loss on it, and then reinstalling the old Onkyo and living with the malfunctioning HDMI out port (to say nothing of the headaches I have been experiencing because of the Dolby Surround and DTS Neural:X upmixing systems, which have been driving me nuts due to our simple 5.1 setup; see my thread about this).
Give the Source Input a try, though.
Oh, one other thing to bear in mind with regard to using this feature: some will argue that you will "overload" the signal by pushing channel levels or the Source Input levels too high into the "+dB" range, ultimately adding distortion and possible failure of the loudspeakers and/or amplifier, but from how I understand it, all AVRs have an automatic compensation system built in, meaning that for every "+dB" step you set, the AVR automatically decreases the overall available volume/power you can use. So, it's not like you're "adding" all these positive stages of gain onto your existing signal...the AVR simply compensates by limiting what power is ultimately available.
Some say this all comes out the same in the end -- and that's true -- but for some of us, myself included, it's about not having to crank that master volume knob nearly to max to get really immersed in sound.