@Empire1
You will definitely hear a difference in sound quality going from the QC15s to either Denon model. Both of them, along with many of the others on your list, will uncover another layer of the music that isn't found on the QC15s or other lower end phones.
As I suggested earlier it may be worthwhile to invest in a DAC/amp to compliment the headphones that you choose. Simply put, an amp will provide power so that the headphones can play at audible (or very loud) volume. A DAC (Digital to Analog Converter) works like a sound card to process the music and send it in a format that your headphones will be able to play. Both amps and DAcs can be colored like headphones, meaning that they can do better at processing certain frequencies and when they output the signal it may not be exactly like the signal that was input.
iPods generally don't have the best onboard DACs/amps, so a lot of people will recommend that you get something that you can use with your iPod. My experience is that out of my iPhone compared to the computer jack, and especially out of my HRT Headstreamer DAC, the sound changes in my Denon D5000. While I do not have a really refined ear, I tend to pay attention to the lower frequencies as I am a bit of a basshead. With the cleanest signal the bass notes sound distinct and there is a clear sense of definition in each one, however, as my source gets worse, the definition lessens and the bassy sounds a bit boomy. he differences are slight, but because I've heard many different headphones in the past 6 months, and because I've grown used to the sound from my DAC, I can tell that the quality is slightly compromised.
If you get one of the Denons (I know this is true for the D2000, also fairly sure the D600 is similar in this regard), you do not need an amp as the impedance for the D2000 is 25ohms (quite low) and the sensitivity is 106db. The impedance levels are also flat throught the spectrum so increase the power will not help in any particular area of sound frequency. Here is an example of the D2000 and the Sennheiser HD598 with a graph of their impedances.
While the HD598 are rated with an impedance of 50 ohms which is also pretty lower, they have a spike in the bass that requires power to properly produce those frequencies.
Anyways, what I'm trying to say is that you don't need an amp for the Denons. A DAC however, is a very wise investment. You will have enough power for the headphones, but the DAC will give you a cleaner signal. Even the budget cheapies will improve upon the iPod. Something like the FiiO E7 with a FiiO L3/L9 Line-out dock to hook your iPod up to it would work. The E7 is a decent performer, but there may be a few better options in a similar price range. The E7 is portable so it will be nicely compatible with your iPod. You can ask around/do some research for additional options.
Regarding the bass between the Denons, I have not heard the new D600, and as someone else mentioned, they are very new so few have really heard them. It might also answer your earlier question from your original post about the price dropping to $400 again soon, which I would say it is hard to know, but likely not. The D2000 are know as basshead cans, but they are very different than the Beats you've tried. It does not overemphasize the mid bass (~100-200hz) but instead has solid punch and extends down very low into subwoofer territory of the subbass. These are the frequencies that will really shake your head. Here is the frequency response graph which will give a general impression of the sound signature.
I do think that the D2000 are a nice headphone and a great first step into headphone land. Hopefully my rambling has helped you and not confused you more
I'm currently visiting family in Beijing and the Great Firewall of China blocked your blog, as they usually do with these kinds of sites. I need to grab my laptop later and fire up a proxy server 
