Shure SRH1540 fits those requirements pretty well. It should be a more comfortable fit for most people than the HP50 with its much softer pads although the headband adjustment length is pretty limited, so it may not be suitable for really large heads. The earpads openings on the Shure also still pretty small, so they may still be uncomfortable to wear for those with larger ears. Also, the cups don't swivel on the SRH1540 which is also a pretty serious issue for me as it makes them harder to adjust properly on the head compared to HP50 and other headphones with rotating cups. Once the Shures are adjusted properly on my head, I find the fit very comfortable and secure. In regards to sound quality, SRH1540 is somewhat better sounding overall than the HP50 to my ears. SRH1540 does have more bass, mostly sub bass, compared to HP50, which I actually don't really like. I think HP50 already has plenty of bass and any more than that is basshead territory to my ears. However, if the low end quantity of the HP50 is not enough for you, the Shures may suit you better.
My problem with the SRH1540 bass is that it's just too much for many recordings in bass driven music genres such as trance, dance and electronic. It's actually a really odd, unusual sound where the sub bass is already melting my brain, while the rest of the spectrum from mid bass and higher sound nice and neutral to slightly laid back. Bass quality is actually very good on the Shures and comparable to the bass on HP50, unless you turn the volume up really high in which case the SRH1540 low end may get a little loose and boomy compared to HP50.
Where I find SRH1540 superior to HP50 is in frequency balance from mid bass and up, resolution and clarity in the midrange, instrument separation, as well as dynamics. I actually feel that except for the sub bass boost, SRH1540 is more neutral and even sounding to my ears than the HP50 in the rest of the frequency range. I don't find the treble to be boosted on the Shures like some people do, and even if it is slightly emphasized, it is also very delicate and smooth sounding without any trace of harshness. I feel that the HP50 is a little dark in highs, relative to the lows and mids, particularly in the upper highs, while SRH1540 has more appropriate presence in the top octave. In terms of treble quality and resolution, I feel that they are quite close, with the SRH1540 sounding a bit smoother and "sweeter", and HP50 being perhaps a bit more natural in tone, but not as pleasant sounding as the Shures.
The midrange is where the Shures come out on top to my ears. HP50 has really good mids and vocals have fantastic body and presence on them, but I also find their mids a tad too forward and slightly lacking separation sometimes, sounding overly bloomy and congested. The resolution is still really good in the mids on the HP50, but sounds just aren't all that well defined and the low mid emphasis gives some vocals and instruments some excessive bloom. In other words while all details are there, the separation and layering of sounds is somewhat lacking. SRH1540 doesn't have this issue and excels at layering and separation, presenting all sounding as clearly defined and free to "breathe" in their own space within the soundstage. I also feel that SRH1540 has more resolution in the mids than HP50 does, presenting more micro detail/texture. In terms of tonal quality/timbre accuracy, I think the Shures are also more accurate. I think HP50 sounds slightly, but noticeably "plasticy" in the mids, which may be due to it's lack of good layering and separation and also because of the low midrange emphasis. SRH1540 doesn't sound plasticy to the same extent as the HP50, but I think they still sound a little smoothed over and still a bit unnatural in tone compared to, say, the HD800.
SRH1540 also does have better layering and separation in the lows and highs too, compared to HP50. Everything just sounds more articulate through the Shures. Dynamics are also better on the Shures - I think HP50 can sound a bit strained at times, not being able to present the full dynamic range of some recordings, or at least not as effectively as the Shures do. Hea
Overall, I think that the SRH1540 is the better sounding, better looking and more comfortable headphone than the HP50, as it should be for the extra cost, but its excessive (in my opinion) sub bass emphasis and also some ergonomic issues (non-rotating cups, pretty small earpads openings) make it fall short of perfection. If they had more neutral sub bass, I would rate them as the best headphone in that price range to my ears, regardless of type, except perhaps for the HE-500. However, the overblown sub bass spoils the tonal balance somewhat and I would choose an HD600/650 or HE-500 over the Shures if I wanted an all arounder with tonal balance suitable for all music. Would I choose the HP50 over the Shures? No, because I feel that the Shures have better sound quality overall and I would rather just EQ the sub bass down on the Shures than take the hit in other apsects of sound quality by switching to the HP50. I also think SRH1540 is worth the extra cost over HP50 becuase it does sound noticeably better overall, has more solid build quality and comes with nicer accessories. And if you are a bass lover who finds the HP50 lacking in bass quantity, then you may not even need to EQ the bass down on the Shures and they may well be your perfect headphone. SRH1540 is my favorite closed back headphone overall, but compared to good open back headphones like a K612 or HD600, they do lack a bit in soundstage and imaging and still sound a bit congested and colored. I would still take them over the open backs for a lot of music though because of their amazing midrange resolution, which I don't believe I've heard yet from any other headphone except for the HD800, coupled with great dynamics and sweet sounding highs.
