Below are comments from Tony Gee of Humble Homemade Hifi. Tony Gee is a loudspeaker designer and interior-architect from The Netherlands. He has been designing loudspeakers since the early 1980's and has published several articles in Dutch audio magazines on DIY Loudspeaker Design.
The AmpOhm FP-CA-AU is a very neutral sounding capacitor that takes a nice smooth and calm approach to the reproduction of music. I found the balance of the midrange very pleasing letting acoustic instruments keep their natural warm tone. Absolute detail and air isn't quite up to the level of an Audyn Cap Plus or Mundorf Supreme but it comes very close although they do need some time to open up so don't judge them fresh out of the box. Spatiality is broad with reasonable depth but if you want "bowling alley" depth go for (the much more expensive) Mundorf Silver/Oil. Compared to the more luxurious Amp Ohm capacitors they are more 2D and a little rough, but again those cost far more.
The AmpOhm PF-XAL-AL 300VDC is very rich in harmonic textures and quite spatious. When fresh out of the box they can be a bit "chesty" or "nasal" sounding but they mellow out after quite some time of use (several hundred hours). Their tonal balance is very neutral and smooth, words like natural and analogue come to mind. Imaging depth is very good but not quite at the same level as their paper-in-wax or polyester-in-oil stable mates.
The AmpOhm PF-XAL-AL 630VDC is basically the same type as the above mentioned capacitor but with a higher voltage rating. The higher rating is obtained using thicker materials and therefore the 630VDC version (65mm) has a larger diameter than the 300VDC version (45mm). Both have the same length (95mm). The 630VDC version reminds me of a can of beer or coke. The higher VDC type is also very rich in harmonic textures and quite spatious. The tonal balance is very neutral and seems a fraction smoother than the lower VDC type. Imaging depth is very good but like the 300VDC version, not quite at the same level as their paper-in-wax or polyester-in-oil stable mates. My review notes also state: black background.
The AmpOhm paper-in-wax aluminum foil type seems a fraction flatter or condensed sounding when in direct comparison to the Ampohm paper-in-oil aluminum foil type. It is a little more direct in its presentation compared to paper-in-oil but more laid-back when compared to the polyester-in-oil types. I also noted less "fleshy" and a tad thinner than the Ampohm PIO. There also seems to be a fraction more emphasis on the frequency extremes giving it a miniscule loudness effect when compared to the very analogue midrange character of the polyester-in-wax type. This is not meant in a negative way seeing the overall high quality sound level of this capacitor, as its character could prove usefull when fine-tuning a loudspeaker in a certain direction.
AmpOhm FE-XAL-AL 630VDC Polyester in Oil / Aluminum Foil
The AmpOhm polyester-in-oil aluminum foil is overall very coherent and neutral. This type has a pleasant midrange-highlight when in direct comparison to the Ampohm paper-in-wax aluminum foil type. The polyester-in-oil is very rich and extremely good in retrieving ambient information such as room or hall acoustics. Music is presented in a natural and full-bodied manner. Compared to the Duelund VSF-Cu copper-foil the image is more "see through". Compared to the Mundorf Silver-Gold-Oil the presentation is richer, especially noticeable on massed strings or large vocal works. On small jazz-trio's etc the Silver-Gold-Oil high-lights micro detail a little more but doesn't quite reach the same level of overall coherence the polyester-in-oil does so well.