You can't go wrong with the M50s imo. I'd recommend you give the Samson SR850 a check out(which are actually a bit of a visual "clone" of your K240, but should sound quitedifferent), as imo they sound better than the M50s and are much cheaper($60 or less). Your K240s are excellent studio monitors, but I understand you want more bass.
Although they hit with a bit less authority in the bass than the M50s, the relative linearity of the SR850s is better for mixing. If your headphones have too much bass, then your listeners are going to end up having too little unless you are really careful about adjusting between differences in your headphones frequency response. However, I'd still probably say go with the M50 as it is the safest route if youre worried about your own enjoyment of the beats in the music, plus the convenience of isolation and portability.
Actually since they are so cheap, you could even buy both the M50s and the Samsons, and keep whichever you feel is best and still stay under $200 US dollars + Shipping, which is less than the Denon would cost you anyway. I'd also recommend the Superlux HD681 as it is only 40 dollars, which I haven't heard but seem to have great bass extension, but their signature seems too V-shaped for proper mixing. If you do happen to buy the Samson, keep in mind that they are particularly succeptible to burn-in and need 100-300 hours before the the bass hits as quick as it can and the mids come into balance with the rest of the frequency.
The HD25-1 II are extremely competent, and very well built. They look kinda retro, but they say I MEAN SERIOUS BUSINESS, imo. They are alot more attractive in person imo, isolate like crazy for something that's not an IEM. I haven't worn them for hours, but I don't find them uncomfortable. I actually prefer their looks to the M50s lol. They are understated yet still have character the M50s don't.
