Well I wouldn't call Q40 very portable headphones, sure they fold but they are still big in size and also they have these wires sticking out above the cups going into the headband that I'd be very worried about with portable use getting stuck in something and when you pull the wires breaks, maybe if you tape them down against the headband/cup it would probably not be an issue but might not look so great then.
Soundwise I think Q40 has a rather even balance between subbass and midbass, these Q40 have lots of both, XB500 is perhaps tiny bit more skewed towards midbass, on Q40 it's fairly 50/50 and the bass response unlike on XB500 doesn't affect the mids/highs at all, the most impressive quantity vs no bass bleed ratio I've come across in a headphone so far, it's so wonderfully separated despite being so strong.
Also I feel the Q40 benefits quite a lot from amping, sure they can be played at 40~45% volume or so ampless out of my Realtek HD onboard sound (compare 28% or so with XB500) but they improve quite a lot regarding dynamicness and bass response with some amping. The soundstage is fairly average, at least instrument separation is a bit better on Q40 but XB500 I thought sounded reasonably airy due to the huge bass vents which helps a lot regarding soundstage as well. The venting is far less on Q40 with only tiny holes at back so it sounds a little less airy but the soundstage size is still good for a closed headphone.
Soundwise I think XB700 would be a good match for you, these are very subbass focused headphone, probably the most subbass focused headphone out there as it doesn't have nearly as much midbass as XB500, even in graphs the XB1000 seems to have slightly more midbass than XB700. The soundstage is fairly the same though, the size is just a little bigger but for gaming use I thought XB500 had slightly better positioning tho but that's not too important for music listening. But then again it's too big for portable use. Another subbass focused headphone which would fit you perfect due to subbass focus + great soundstage would be BeyerDynamic DT770 Pro/80 ohm but this one would need at least a little amping to sound to its full potential and it's also a bit on the bigger side but it's VERY sturdy though.
Edited by RPGWiZaRD - 1/19/12 at 1:02am