Though bass guitarists -- such as the aforementioned John Entwistle -- have used the Marshall amplifiers for bass guitars, they are most often used for normal guitars, and this informs the Major 50 FX's sound.
I used a variety of sources, including an iPod Touch, a Samsung Galaxy S and a HiFiMan EF2 USB DAC. These are most likely to be used "on the road," but PC listening is a possibility, especially as many ultraportable PCs have a combined headphone/mic output now.
Given the amp's history, it goes without saying that the Marshalls are best suited to rock music, but I soon found the bass wasn't overwhelming on non-bass-oriented tracks; these will also replay acoustic or classical music well.
Switch to a bass-heavy track, though, and this changes. Given a copy of David Byrne and St. Vincent's "I am an Ape" to hold onto, the Marshall's bass seemed to linger and push all of the other instruments and voice into a tight space. The headphones lacked the bass attack of the the more articulate (and more expensive) Bowers and Wilkins P5s. Give it a monster bassline, such as "Life" by The Beta Band, and there is still some upper-register articulation available despite the potential heavyhandedness. While a little buried under bass, the glockenspiel part toward the end of the song sounded crisper than on the B&W P5s/P3s and Grado SR60is.
Noise isolation was decent as the earcups closed over my ears and only needed to turn the volume up once on the subway when a particularly noisy train whooshed past. Likewise, using them as a phone headset worked fine with good call quality on both sides.
With the price-comparable Bowers and Wilkins P3s on my head, I found their sound to be pleasant if a little dull, while the Marshalls were involving without being too taxing. With the P3s, "Yet Again" by Grizzly Bear sounded constrained and lacking in high-end detail, while the Marshalls had more air and a sense of space.
But this air can be at the expense of true treble detail. The Marshall's treble could be a little splashy at times, with cymbals sometimes sounding more like tambourines, but this was usually kept in check.
If you're looking for a pair of headphones that scream "ROCK" in an throat-ravaged falsetto -- inside and out -- then the Marshall Major 50 FX are the headphones for you. They're fairly restrained in a rock way: more Angus Young than Brett Michaels, and they have a fun, vibrant sound that would suit fans of most genres of music. With a $120 saving over the Bowers and Wilkins P5 and a more lively sound, these are a very good set of portable headphones for the money.