I'd appreciate it!
Overall smoothness, extension (in terms of bass and treble), dynamics, sibilance (and general female vocal performance), timbre and imaging are things that are important to me.
I own the R70X so can reference to it, but I am not usre how accurate the reviews portray the Zeus and the Planar.
So I'll start off with broad comparisons and then get into more specific details.
In terms of technical performance, I'd say it's something like Avantone > Zeus > R70X. But the gap is not so wide that I'd say they're in different tiers. I don't consider any of them to be upgrades of each other, except maybe the Avantone being a slight upgrade because of the planar advantage.
In terms of tonality, the Zeus has a decently V-shaped sound with a lot of warmth and bloom to it. The R70X is pretty much neutral while the Avantone is the darkest of the three headphones, but still within range of what I'd say is the dark side of neutral.
Out of the three, the Zeus is the most different sounding headphone. That V-shaped tonality is very exciting to listen to and "grand" in a way. The bass elevation is more in the midbass, so it's harder to make out the subbass here compared to the other two headphones. It's also not a very hard-hitting headphone in terms of slam, the bass is more like a wave than a precise punch. But this gives vocals and instruments a lot of power and body behind them. When I go from the Zeus to other headphones, they sound a bit "small" until I get used to the sound again.
But despite the bloom, the Zeus somehow maintains good clarity without sounding unnatural. The treble is a bit spicy though, I find it borders on being sharp and edgy at times. Nowhere near as bad as a Beyer, but it isn't as smooth as the R70X. It's also surprisingly fast for a warm headphone, I think faster than the R70X, sounds are tight and well-separated even during busy passages.
The soundstage isn't particularly large, but it is more spacious sounding than the Avantone or the R70X imo. The imaging is also pretty good, though I think the R70X is still a bit more precise there.
For female vocals...I don't think the Zeus is the best choice. That bloom and warmth kind of makes female vocals sound a bit too "thick" for me, robbing some of the delicate touch they can have. And despite vocals having good presence and clarity, the elevated bass and treble still takes some focus away from them.
Now moving onto the Avantone, this is a darker headphone. It rolls-off in the treble more than the others and that gives the sound a "veiled" quality to it. Things like drum hits, cymbals, synths, etc sound blunted and rounded-off. There's no snap or edge to them and it's harder to make them out in the background. But on the other hand, there's zero sibilance and the shape of the upper mids/lower treble is pretty good. So you still have good clarity and presence in stuff like female vocals and strings, they're just a little less "in-your-face" than the R70X or Zeus.
The bass is pretty good on the Avantone, you can make out more information and texture there than on the Zeus or the R70X. However, for my ears, there's still some subbass roll-off on the Avantone, though I think I'm just not getting a great seal with the velour earpads. I found that the R70X had a little more rumble and physicality to it.
The Avantone is less fatiguing than the Zeus, but I wouldn't say it's "smoother" than the R70X, despite sounding darker. The reason is because I find the R70X has a kind of Sennheiser-ish presentation, where sounds seem to blend a bit into each other. It's not muddy or congested, but it's easy and pleasant to listen to. The Avantone has some of that blended quality, but it still retains some dryness from that tight, well-separated planar sound. So while it's smoother-sounding than other planars, it's not to the extent I'd sit back and relax to it.
For soundstage, I'd say the Avantone is a bit smaller than the R70X. But it was hard to tell sometimes because both had great imaging that kind of changed depending on the mix. But the Avantone still has the more precise imaging imo, for music at least. Sounds are layered so well that you can always tell them apart even if the music becomes a wall of sound. And "distance" especially is something the Avantone does really well, it's surprisingly easy to tell how forward or distant a voice is compared to other heapdhones I've heard.
For female vocals, the Avantone is better than the Zeus. Because the shape of the upper mids is good, there's still plenty of clarity and presence to female vocals. I find the midrange resolution on the Avantone is also quite good, so vocals and instruments have great texture. Combined with the excellent imaging and that slightly "blended" quality, vocals and instruments have a "lifelike" quality to them. But the timbre is still a bit off due to the darker tonality. For vocals in particular, I also hear a bit of a honk that makes them sound a touch grainy.
But without EQ, the R70X still sounds better with female vocals because it has that additional clarity and sparkle the Avantone lacks. With EQ, I think the Avantone edges out due to its more "realistic" presentation (though I think the R70X will still be a smoother listen).