One thing worth trying before you sell the 840s is a graphic EQ and you can turn down everything but the lower frequencies but IMO if you have to EQ a pair that much it begs the question of why didn't you buy a pair that suited you in the first case. Now I've only heard the DT770s so I can't really offer much advice regarding the others but a quick google turned up
this (bear in mind these aren't the pros) and I am sure it's been discussed to death on Head-Fi so just have a look around. In terms of open vs. closed, if like you say you'll be using them in a fairly quiet environment then open's probably the best bet as they're typically considered more natural sounding compared to closed cans. The 770s are closed, 880s are semi closed/open and the 990s are fully open back. The DT770s have a U shaped sig in that there's an emphasis on lows and highs with sucked out mids, the DT880s are widely considered to be the most neutral sounding out of the lot of them and the DT990s supposedly take the DT880s neutrality and combine that with the well extended, thumping bass. As you say you value bass but the rest of the spectrum is important then I think the best option for you from the Beyers is the DT990s. Of course, the best way to tell would be to listen to them yourself but that's not always possible I know. Then there's the problem of which impedence to buy and that largely comes down to what you'll be driving them with, I think the pairs with greater impedence offer sonic improvements but that's just something I've read. Other options worth looking at are Ultrasone who make bassy cans, the HD650 which is a little more expensive than what you're looking at but has reportedly great extension or the HE-400s.
("get the hd650 anyway! In fact, buy the HE-400 as well! And buy a better amp and dac while you're at it!")
