I’ve never owned or heard a 1990 personally, so I can’t really comment on it. I buy everything I review, and I don’t usually spend more than $350 on a pair. The 1990 is based on an entirely different driver platform than the 990, and has a sound target at studio mixing/monitoring applications.
Metal571 has a great review of them.
As to the differences between different Ohm levels of Beyer models….while they sometimes have slight sound signature differences, Beyer tries very hard to tune them all identically. They want to have a wide range of options so that you can pick the resistance that best pairs with the equipment you already own.