Thinking of a socalled recap of my Beovox S45-2's, I wondered if any new comparable capacitor will be an upgrade, or is it worth searching for a special brand/ sort of capacitor? 

Every new capacitor is better than a 20-30 year old used cap. So, just replace them and you have a better result.

But: if you replace a electrolyth cap by a foil type, there is a huge difference in quality. You will never ever replace a Wima MKS or MKT or Intertechnic or Audyn again. A foil type like the Audyn MKT will last forever and will never ever go defect. They last a lifetime and longer.

And Audyn has a very nice sound, the probably best what you can get for money.

I recapped my S45-2s within the past two weeks.  Martin, handle of "Dillen" on the forums, can hook you up.  While at least one member of the forum will tell you the caps need about a 200 hour burn-in period, I noticed an immediate (positive of course) difference.  I've probably got 50 good hours into them now, and I'm a very happy S45-2 owner.

I was prompted by a specific problem:  at just higher than moderate listening volume level, one midrange would begin to have diminished performance.  The best way I can describe it is the midrange driver would become raspy.  I had owned the S45-2s for about 2 years, but must confess, they had been sitting in a box most of that time.  And I rarely had listened to them, and never very loudly.  For all I know, they had been "bad" the whole time I owned them.

Anyway, they are now part of the system I use the most.  (The rest of the family is in the living room watching and listening to a different system.)  And they are a wonderful pair of speakers after the recap.  Oh, and the recap solved the midrange problem.  I did get a spare midrange from Martin/Dillen at the same time I got the cap kit, but my original midrange was OK all along.  It was a cap failure.

Read this thread for another happy S45-2 recap story.