I've owned two variants of HE1k (v2 and SE) for 3-4 weeks respectively years ago. But never been able to compare he1k se and caldera side by side. So, please bear with my recall bias. Just for another reference, I've been owning Susvara for 1.5 years (and preferred Susvara to any variant of HE1k by far). TLDR is that Caldera is tiny bit more preferable for me than Susvara.
HE1k (anything below can be either pro or con depending on tastes)
- Brighter
- Relatively a little more sibilant
- Some hint of artificiality in highs
- Lighter bass
- A bit overly wide soundstage + slightly distracting spatial cues
- Clearer mids and upper mids
- Details are more straightforwardly separated with mild low-level loss
- Squarer transients
- More vivid and sharper presentation
Caldera
- More refined across the entire spectrum
- More elegant presentation
- Soundstage width-depth ratio is more believable
- Far more natural and convincing timbre
- Better nuance retrieval
- Low-mid-high balance is nearly perfect
- Delicacy in bass is fantastic
To my ears, I felt HE1k used to oversimplify things a little compared to other TOTLs. Susvara fixed this elegantly. Caldera is on par, equivalently good with a little less planar-ish style. Overall, I think HE1k deserves its used/deal prices while Caldera is worth its full asking price.