I'm digging out this thread to say that after one year with my PentaVox speakers, I decided to follow other members' advice & replaced all capacitors on midrange & bass-treble filters, with :
Mundorf electrolytics 68 uF/100 VDC (none was available in 400 V to match the original dimensions)
Audyn Cap MKP polyprop. 5,6 uF/400V
Jantzen Cross cap MKP 8,2 uF/400V
Jantzen Cross cap MKP 22 uF/400V
Jantzen Cross cap MKP 27 uF/400 V
Although they were all larger/bulkier than the originals, I managed to solder them all on the boards, and after the first 10 minutes, playing music with the new caps, the 3 of us in the room were in awe...That cost me EUR 75 for the caps, and was worth every cent!
The Pentas sound alive again, the treble level adjusted to +3 on my BeoCenter 2 went down to +2 (I didn't replace the original tweeter, but admit that if set to 0, the treble sounds like the original Penta of course, which means " a little bit behind" the rest of the range).
The "new" sound is really full, my speakers are 3.6 m apart from each other, and there is a real wall of sound, the bass is still impressive, but now the mids are lifelike, and treble are..treble!
The most impressive change was in the stage that is no longer "left/center/right", but much more "far left, left, mid-left,center,mid-right, right and far right" when you're playing good recordings & classical music. So I can only highly recommend this cap upgrade, especially on the Pentavoxes & Mk 1 BeoLabs of course.
Now that the mids have been refoamed, back connectors soldered with silver, all caps replaced, what's left to do technically speaking? Replace the treble unit? Or simply adjust the treble level on the BeoCenter/Sound like I'm doing now?
My next step for the final touch will be cosmetic: I need to replace the frets! They are 25 years old, after all!