the pro 900s have massive bass punch but not as much sub-bass, though its still strong. the bass isn't clean like the sennheisers though, its more muddy/boomy. its a fun headphone though. also note that it has recessed midrange and a bright metallic treble. so if you're looking for a magic midrange, they won't have it.
i would check out sennheiser hd 650s or maybe beyerdynamic DT 990 (preferably the 600 ohms), these have a straight cord btw, only the pros have the coiled. anyways regardless of wheter u get these or stick to hd 600s, get an amp. no amp = no bass. just check out the 100$ schiit magni or something. also, highly recommend the hifiman he-400. punchy with great midbass and a good overall sound.
i would not recommend the denons since they aren't as clean sounding as the sennheisers, and it appears that you aren't looking for sub-bass.