I would not get the sundara headphones if you think the 6xx are lacking bass. Sundara has almost no bass. 6xx are entry level. Lower your expectations. The reason why the 6xx are considered so good is because it is often the first pair of headphones that people are introduced too.
I would not buy anything in the 300 dollar range if i were you. I would buy an amp as it seems you have a dac. I would than look for a pair of headphones you could save for for like a year or something.
I would not get the sundara headphones if you think the 6xx are lacking bass. Sundara has almost no bass. 6xx are entry level. Lower your expectations. The reason why the 6xx are considered so good is because it is often the first pair of headphones that people are introduced too.
I would not buy anything in the 300 dollar range if i were you. I would buy an amp as it seems you have a dac. I would than look for a pair of headphones you could save for for like a year or something.