Sstalwar, you can spend more for the Duelund, but they will not sound as good or certainly not sound any better than the Jupiter copper foil cap. I have used both in numerous electronics, tube amps etc... This has been my experience.
I find the Jupiters to be a bit more lively, better in micro details, and not as tipped up sounding in the upper mids. This has been my experience with both these caps in tube amps.
It is reasonable to suggest that these caps will perform the same way in your tube amp. Both are very good caps indeed and you can't go wrong with either, but the CAST caps are lots more money.
I find the Jupiters to be a bit more lively, better in micro details, and not as tipped up sounding in the upper mids. This has been my experience with both these caps in tube amps.
It is reasonable to suggest that these caps will perform the same way in your tube amp. Both are very good caps indeed and you can't go wrong with either, but the CAST caps are lots more money.