So Crimson vs. U4s comparisons, correct?
The U4s has a more agreeable tuning and a neutral-sweet tonality. The Crimson's tuning is more U-shaped with an elevated bass/treble and its tonality is more neutral-warm. The Crimson's treble can be a bit harsh to some people (tip-rolling to the rescue).
The U4s is no slouch, but the Crimson is more dynamic, resolves better, images better, and has a much larger soundstage. It's more similar to the U12t technicalities-wise than it is to the U4s.
Yes. Crimson and U4s are different enough to keep both as complimentary pairs.
The OG Helios had thin mid-bass (to some). The Crimson's is elevated. If you go to my squig site, smirk.squig.link, I uploaded Crimson's FR. You can compare it to the Helios and Helios SE's bass. I don't think the Crimson's bass shouldn't come across as more lean than the U4s' by any measure.
The bass quality on the Crimson is better than both the U12t and the U4s IMO. The Crimson's bass is more textured than both and has better decay than the U12t's bass. It's really satisfying.