Always use a bypass cap in parallel with your other cap. In series, the total capacitance will always be lower than the smallest value cap.
For example, if you put a 1uF cap in series with a 100uF cap, the measured value of both of them would be around .99 uF.
Back to the Duelunds ; I compared an Aluminum Duelund to a BAT cap to see if there was an improvement. BAT uses a very similar cap to the Jensen Copper foil in oil. The BAT caps have copper leads, whereas Jensen uses silver leads.
The BAT caps were the best, followed by the Copper foil Jensens, then the Aluminum Duelunds. The Duelunds were too bright and not as tonally correct.
These test were done on signal level and not speaker level. I mention this because speakers normally sound better with the brighter caps, rather than those which are darker and richer, which to me, are preferable on line level and signal coupling.
So I leave it open for the Duelund Aluminum on speakers. But based on all that I have read, the Aluminum Duelunds have never outperformed the Copper.
rilbr