Looking to put this review up either tonight or tomorrow. I apologize for the delay, it appears life did a little catching up with me recently :)
Anyways, to help you out I have been A/Bing the HD448 to the PX 360 a bit to help give you some personal insight into your decision.
Initial impressions are causing me to confirm my soft spot for the HD448. I will do much more listening to give you a more accurate and greater sample sized perspective, but as right now I am not convinced that the PX 360 is a substantial step up from a shear sonic perspective. Obviously, the PX 360 have some other things to offer than the HD448 do not - collapsible, smaller, lighter (although the 448 is very light as it is), and of WAY better build quality. The PX 360 are quite durable, and feel solid in the hand without the worry of weight. Additionally, the PX 360 isolate VERY well for a passive system... I'd say the HD448 are a notch above average at isolation and the PX 360 are quite excellent at it. The PX 360 are also easier to drive, the 448 benefit from an amp much more so than the specs suggest, and the PX 360 barely showed any differences out of my e7 or e9. The 448 on the other hand paired a little awkwardly with the e7 and very well with the e9 (still much better than right out of the iPod DAP in both cases).
The PX 360 certainly have their place, do some things outside of sonics quite well.... but I have not decided quite yet if I will be holding onto these or not as I feel the 448 might be too close in comparison to justify the upgrade for me.
That being said, their signatures are VERY different - more on this when the review comes. Since I will be writing a review for the PX 360 alone (and not focusing on the 448 comparison) you'll have to switch between my reviews and decide if you agree with what I had to say about the 448.
Initially, I am noticing the PX 360 to have that extra punch the 448 is missing, but it gives up some nimbleness in the lows as the cost. I imagine the bass to be a full, yet cold type low end. It took some getting used to, but I am finding I am okay with it. These PX 360 certainly have life in the low end, maybe even a little too much for my tastes. The PX 360 are a little more treble oriented than the 448 as well, and while this helps bring a lot of life to some tracks, it can cause others to be fatiguing. IMO the mids in the 448 just crush that of the PX 360.... the 360's mids are certainly there, but much darker to me, and combined with the lively highs almost gives it the U character I don't care for. The mids of the 448 are just sooo smooth, especially at that price point.
More on this as I A/B some more. I don't mean to bash the PX 360, they have a lot to offer... just might not be my taste (still deciding)