I agree with Volleyguy's hierarchy for the most part. Improving couplings caps in the upstream electronics makes the biggest difference. I would probably tweak the ranking just a bit. Going from biggest improvement to less:
1. Coupling caps in linestage
2. Coupling caps in phono stage
3. Resistors in key locations of phono and linestage
4. Capacitor in tweeter crossover
5. Coupling caps in power amps
6. Resistors in tweeter crossover (if any)
7. Inductor in woofer crossover
8. Capacitor in woofer crossover (if any)
9. Inductor in tweeter crossover (if any)
Of course, the degree of difference that a cap replacement makes depends on what type of cap is being replaced and by what. You can expect a much bigger difference replacing a Solen with a CAST than a V-Cap with a CAST, for example.
It is often said that no capacitor is perfect and they all have some type of sonic failing. That is certainly true. Yet I don't recall ever reading any description of the Duelund's failings. There's no question the Duelund CAST copper sounds very good, and we tend to focus on all the things it does right but it isn't perfect.
Based on my experience with 3 pairs of CAST caps (.47 at 630v, 1.0 at 630v, and 7.5 at 100v), and having tried the high-voltage caps in several locations, both as bypasses and solo, I would offer this one complaint. The Duelunds don't have enough high-frequency life and sparkle. They sound just a bit too smooth in the upper mids and treble and not quite as airy and resolving as I would like. Now you won't hear those areas as problems if the CAST replaced a Solen. But compared to a 1.0uf Sonicap Platinum, that's what I hear. The SP has more life, more air and more finely resolved detail in the upper highs. The Duelund sounds better overall than the SP---better balanced tonally, more detailed, more neutral---but I do miss the special qualities of the Sonicap Platinum in the treble. (Note: these comments only apply to the 1uf SP, not the smaller values).
What do the rest of you think about the CAST caps? Do you hear any negatives?