Today I got to pick up the alpha pads which I wanted to use with the hifiman HE-400.
Turns out, they do not sound good with the HE-400, very muddy sounding next to the J$s.
When I heard them on the GMs my first impressions were basically the same as with HE-400, very muddy sounding. Then I switched back to the oval pads and I have to say it's not as muddy sounding as I first heard.
What do the alpa pads do to the sound? Bass increases with about 5db, it's substantial. Treble sounds a bit smoother, and the 'glaringness' of the sound is either lessened by a decrease of the upper mids or is masked by the boost in bass.
Pro's: More bass quantity, with about the same quality. Upper mids are tamed a bit.
Cons: A bit less detail in treble. A thicker sound due to the added bass.
I'd say this sound is slightly more enjoyable than with the oval pads because it completely terminates the tinny sound I was complaining about.
Interesting fact, the alpha pads completely lack damping holes. I'd say adding holes could only help the sound.