Interestingly, this can work in the opposite way on some headphones (at least on my head). My HE4XX, Edition XX, M1060, and ESP/95X all have a noticeable bump in bass (5-10 db) when they are aren't sealed well all the way around the ear. Most of my other headphones are the opposite, however, so it doesn't surprise me that the HF580 loses bass without a good seal. And I agree with you 100% on the Sendy pads. I would speculate that a good chunk of that $600 Aiva price is R&D cost on those pads. The materials, shape, and fit are completely intentional to get the most out of this driver. I highly doubt they would work that well with any other driver.
I wouldn't try putting a price to r&d. $600 was because they got to market first and the build quality is honestly top notch, worth 600 more than some 1k+ headphones on that alone. even the wood cup clones don't have a grill as attractive, headband as good or cups as nice.