I received the CAST 0.56uF caps for my preamp and installed them last night, then spent about 4 hours listening. The main difference between the CAST and the Mundorf Supremes I was using is in tonal balance. The CAST has more rounded and fuller midrange, low midrange and bass than the Mundorf. The treble is about the same so far.
I do feel that the Duelund sounds better due to the better tonal balance. Sax sounds more like sax, voices are especially nice.
I'm using the 200VAC CAST, not the 630V "electronics version". They are smaller and just fit into the preamp - the larger 630V would not have fit without extensive modification. An added benefit is that they are also less expensive (but still close to $600 per pair). Both Duelund and PCX assure me that these can easily handle at least 200V DC, even though they are marked as 100VDC on the side of the caps; my preamp runs at 175V DC on the plates.