Mini 550 and V6 comparison, the V6 definitely has a brighter/more forward presentation than the 550. After some A/B, definitely noticeable; the mids and treble are more pronounced on the V6. The 550 are a bit more laid back, but the treble is still definitely present, a tad lively, definitely not muted.. The 550's bass though is stronger; more solid, goes deeper. The K550 is definitely the warmer sounding phone of the two.
Versus the Etymotic hf5, again, 550 is warmer. The Ety's bass is more comparable to the Q701, both seem to be quite brutally neutral, in all frequencies, so if a recording has bass, they'll reproduce it, but won't add anything to it, which might often sound 'lean' to some. The K550 definitely adds bass, and has a warmish tone due to it, much more so than the HF5 and Q701. As I said, they're rather laid back, because the mids are a bit further back, but the treble is still present. Bit less so than on the V6, but not that much. Still, the AKG K550 has that AKG sound, fairly balanced on average, might have more bass, but still sounds balanced, go figure!
Overall, I'd say that the Ety HF5 and Q701s definitely have that 'studio monitor' balanced/neutral sound. The V6 is definitely brighter sounding, with some bass bump/hump/boost. The K550 has more bass, extends deeper and is warmer than the 3. Its mid are a little bit more laid back, and with lively treble, but not as much as V6, probably closer to the Q701. Still sounds quite balanced to me. Hope this helps