I'm not really familiar with the C-21. I did a quick google and to be honest it doesn't "look" like anything special...but then many may say the same about the Cursa.
I'll give you what little help I can. I currently have a Rega Hal preamp driving Rega Exon mono blocks. I've had this set up for almost four years now. Previously I was mainly into vintage, but never had vintage seperates, just receivers (Sansui 9090db, Marantz 2325, McIntosh 1700).
The Hal was the predecessor to the Cursa, and I believe somewhat more expensive. That being said, when I bought mine, I bought it NOS (it was around 10 years old, but came from the warehouse of a former Rega distributor). The dealer I bought it from also had a NOS Rega Elicit, which was their top end integrated from the time. He wanted $500 for the Elicit and I believe $1,500 for the Hal/Exon combo (keep in mind that the Hal/Exon combo listed for ~ $4,500 in Canada in the late '90's). I listened to both hoping to not notice mush of a difference. The Hal/Exon combo blew us both away in comparison to the Elicit. He let me have the Hal/Exon combo for $1,000 since it was open box NOS as opposed to the $1,500 for the unopened combos he had.
If the Cursa is as good as the Hal, then it should be a large step up.
Mike