I've got to agree. I spent a couple hundred on my caps and every time I sit down and listen, I'm glad I did. I agree that they are expensive, and say what you will, but the buffer with the Duelunds produces a silky, more tonally correct sound on my system. Moreover, the copper 3.3 VSF are $219.99 not $300+ (that's the black, I think).
Honestly, I probably wouldn't buy the CAST for for the buffer, as the big deal is the heavy casting to help against resonance. Which would really be important inside a speaker cabinet. Probably the best for the buffer application would be the black series, which have a light "casting" as it were.
I bought the Duelunds because I figured I would use them for awhile and then when the balanced version with the HT bypass came out, I'd transfer them and buy two more to go in the new one (it takes 4). I guess all I can say, is that for me, the modded buffer along with my 9's produces out of sight sound.
To be fair, I have heard that many people like the sound of the Clarity Cap MRs. I have heard that they sound very detailed and very clear. Pretty much like possibly better sonicaps (I've never heard them myself). However, asking around, I have been repeatedly told that if you are looking for a tonal upgrade and silky (not rolled off) mid-highs, go Duelund copper. That's what I did and I've been very happy.