EereeThey’re known for not being overly bass heavy. Depending on your listening preference and what you’re used to, it’s likely simply just that. A long time ago I tried a K702 pair as my first “reference” phones and ended up returning them because I couldn’t stand how bright and lacking bass they were. However I now am ordering these again after a while of getting used to other “reference” quality phones. IE - I’ve learned it’s not “all about the bass.”
Eereenot sure if by "terrible" bass you mean muddy or just not present enough, but for reference, with my setup (see below) the k7xx has plenty of bass, though it doesn't sound super tight to my ears. compared to both the 6xx and 4xx, the k7xx has more bass (a LOT more), but it's muddier. not unpleasantly so, though. (for what it's worth: i just got the k7xx this past weekend, so not even 10 hours of break-in. that said, i don't subscribe to the belief that break-in matters for most cans.)
keep in mind that the k7xx are open-back, so they will sound airier and less bass-heavy compared to a lot of closed-back headphones, especially "consumer" cans (which often overemphasize bass to the point of absurdity).
and they definitely need an amp, also.
good luck, hope you end up enjoying them. :)
setup: objective 2 / odac, switched to high gain, knob at about 10 o'clock; windows volume set to 50%.