To the OP,
First off ALL Ultrasones need at least 300 hours of burn in. This is due they have very stiff drivers. You can do this in a variety of ways. One way is to use pink noise or a frequency sweep that is repeating. The other way is just play your own music on a continuous loop. The music method is the one I used to burn in my Ultrasone PRO 2500. With burn in your headphones should develop more bass and that bass will be even more refined. Also when conducting burn in make sure you don't stress the drivers too much. Turn the volume up about 2 notches above your normal listening level and don't burn in for more than 24 hours at a time. After 24 hours give the headphones a 1 to 1.5 hour break and then resume.
Second - Ultrasones especially the PRO ones need an amp of some sort. Ultrasones work extremely well with solid state amps. I personally use the iBasso D4, which is a great amp/dac combo. An amp will also bring out the bass you're craving for.
Third - I see that you are trying to EQ the PRO 550. This may help, but you are gonna need a really good EQ in order to get results. I suggest that you stick to getting an amp if you are going to keep the PRO 550 and get that bass you're craving.
Fourth - I also see that your intention is too find a headphone that doesn't require an amp. In this case the PRO line of headphones isn't for you. Go for the HFI line. These headphones really don't need an amp and sound great once burned in with at least 300 hours. The 580's that you mentioned should fit the bill nicely. But the only way to find out is to order them and test them out despite your concerns that they might not be what you are looking for. Currently they are 150 on amazon.
Hope this helps,
the 550 are not open correct? I have the 580, they isolate very very well and have a great soundstage, not so much outwords as more of a dome effect, it comes in hand in games. also no ultrasone really needs an amp but like all headphones they benafit form them.