It's interesting that the V6/7506 came up in the midst of a Beats discussion, since I'd say it's the closest the headphone world had to Beats before Beats was but a gleam in the greedy eye of Noel Lee*. I remember almost 15 years ago reading through pages and pages worth of forum people and consumer product reviewers praising the V6 variant**, making it seem like the greatest thing since sliced awesome. It got recommended left and right. IMO, it's honestly nothing special. Pretty good for 1985 or so when it was new, likely durable as all hell (the garbage-bag-stretched-over-cheap-foam earpads notwithstanding), but not really competitive with newer options.
I'll give my impressions for the sake of discussion, since I've heard several of them on numerous occasions. The overall tonality leans toward being midrangey, with a persistent edge and a noticeably rough, "dirty" character. The treble drops off a cliff above 10 kHz (likely why my friend, who has a 7506 he's over the moon about, said my HD 600 sounded "tinny" to him). The upshot is that everything sounds dry and a bit like the sonic elements are dissociated cardboard cutouts. The lower and mid treble are elevated, lending a sort of "spitty" character to the upper harmonics on vocals, and a strange, unnatural sound to cymbals. They don't shimmer--it's like bursts of metallic noise, and it's all attack and no decay. If the upper mids are a bit pushed forward on the HD 600, they're excessive on the 7506. It gives the sound that typical "lo-fi" character that comes with lack of extension, kind of like listening to everything through a midrange driver. Or a clock radio. Despite this, the bass extends surprisingly deep, but the upper bass to lower mids transition has a noticeable dip, meaning voices lack depth and the bass sounds isolated and hollow. It's overall a great example of a headphone that draws a nearly perfect line on a diffuse field-compensated graph and yet sounds pretty far off the mark.
Much better, IMO, is the Shure SRH440. It's not perfect, and it requires a bit of modification for both comfort and sound reasons, but it's a lot closer to the HD 600 in raw sound quality than others I've heard (the aforementioned 7506, the infamous M50, even the more expensive HP50). In stock form the baffles tend to dig uncomfortably into the ear after about half an hour, and the sound has a persistent tizzy edge that no amount of EQ can solve. The treble spike is also a little too powerful, leading to fatigue on brighter material. The trick is to affix felt over the drivers (though not over the bass reflex ports), which eliminates the comfort issue, softens the treble peak, and cuts down on the tizz. This last one is likely because the felt front loads the driver and damps the mid treble ringing somewhat, leading to a cleaner sound.
It's still not on the level of the HD 600, but it's a good deal better than I've heard elsewhere, and for $99 it's a pretty good deal, IMO. The overall sound is slightly V-shaped, but with strong presence in the upper midrange. Not quite as neutral as the HD 600, but very nicely balanced, and by virtue of this an excellent all-rounder. I've gotten quite a bit of enjoyment out of my pair over the past six years, though it's not the most durable thing out there, and the headband requires further modification if you, like I, happen to find it uncomfortable.
*Or, if the rumors are true, his son, who sold the cow with the milk when he drew up the deal with Dr. Dre without any input from daddy.
**The 7506 variant is wired slightly differently and utilizes a gold-plated connector rather than the nickel-plated one on the V6--the two headphones are otherwise identical, but that hasn't stopped people claiming one or the other sounds noticeably different/better
My experience with the MDR-7506 and HD-600 is really different, to me the MDR 7506 has a slightly recessed mid range making it good for listening at very low levels, its like it has a built in loudness switch like some old hi-fi gear. A lot of my friends and family that have very little or even those with some exposure to decent audio gear will almost always chose the MDR-7506 over the HD-600, and I suspect its the same reason with Beats and Bose, its all about the bass, to most not in these pages the more bass the better the headphones will appear to sound to them. Another reason is if I let someone listen to the HD-600 and MDR-7506 on a smart phone like a Galaxy or iPhone they will always like the MDR-7506 over the HD-600 due to the better dynamics and bass, the HD-6XX and most better headphones need a decent amp to be driven properly but there are exceptions. And if we go with a budget of $75, hard to find better than V6/7506, but if anyone knows of others in this price I wouldn't mind hearing about them.