OK, the moment of truth. Finally got my hands on Dekoni pads & ring combo. So here is Dekoni sheepskin pads(with and without the attenuator ring) vs Fostex stock pads comparison. I have measured them extensively using my THX00 Ebony on a Head-And-Torso-Simulator.
Comfort & Fit
While the stock pads do not provide much cushion for not being thick enough (and to make things worse their compliance differ slightly from unit to unit), Dekoni pads are filled with high quality memory foam, which give the listener great comfort even when glasses are worn. They might cause a bit of stuffiness in summer due to their tight fit, but I'd still choose Dekoni's over Fostex's, period.
Dekoni with the ring VS Stock Fostex
First of all, it is vital to mention that their sub-bass performance are IDENTICAL. And here's something interesting: Dekoni ear pads pull down resonances in the upper treble above 3.5 kHz slightly downward, shifting the signature(or problematic) peak at 10 kHz to about 9 kHz, due to slight angle&depth difference in how the pads place the driver on to the ear; But keep in mind the stock pads are curved and the Dekoni pads are made flat, while both are still angled. Since I personally do not prefer the shrilling peak at 10 kHz, this is definitely an improvement for me. The tonal quality becomes somewhat similar to that of Sony MDR-CD3000, but with a lot more sub-bass response.
Dekoni without the ring VS Dekoni With the ring
The attenuator rings(aka Fostex acoustic guides) placed inside work like a low-shelf filter that affects below 1 kHz. They boost the sound pressure level up to 7 dB @ 20 Hz. This, in my opinion, is a BIG difference and is caused by:
1. Driver having to deal with less amount of radiation impedance due to decreased volume in front of the baffle
2. Acoustic guides perfectly sealing the dense foam ring situated at the driver's opening
Acoustic leak becomes extremely well-controlled, leaving the porous foam material as the only controlled leak between the pad and the driver. The other important element, acoustic leak between the head and the pad, is still present, but the resulting sound pressure level difference below 1 kHz is now substantial, sustaining a lot more bass even with a good amount of head to pad leak. The Use of attenuator rings is generally recommended for listeners with a normal amount of hair & anyone with glasses, but not for those with a very short hair or a shaved head. Since their head cause less amount of acoustic leak, they may find the sound with the ring overly bass oriented. (Or not if you are a bass-head)
Dekoni sheepskin pads are far better than Fostex's stock pads. The only comparable pads in quality are $100 JMoney pads(well-matched early production pairs). Bass response remains more or less the same compared to the stock ones if the attenuator rings are utilized. Treble signature above 3.5 kHz changes a bit, but with less(or more depending on how you perceive treble) fatiguing sound. Highly recommended if you are looking for a quality replacement pads for your THX00, Emu, or Denon.