I have the HD 558s and did the foam mod. IMO, the foam mod sounds a LOT better. Bass is fuller, and you can still hear the detail. Midrange is more forward too.
I also have a set of Denon AH-D2000s. The open Sennheiser sound is interestingly different. Even if I listen at loud volumes, the sound is sort of 'laid back'. Voices sound sensational, much better than the Denons, but everything else has this airy quality about it. I would definitely listen to ambient, progressive, classical, or "chill acoustic guitar" style music on this.
However, for high energy music, I definitely prefer the Denons. I think I've grown accustomed to having bass 'rumble'. The Denon's bass acts like a subwoofer, it feels weighty. The Sennheiser's bass is just there. Accurate and "there", but it doesn't punch you or rumble you (in subjective terms). Also, a snare hit in the Denon's will pop in your head. To me, that is an appealing sound: I want to feel like the drummer is thwacking that snare with feeling. The Sennheisers are more analytical, you don't "feel" the snare. Also, the Denon's have this 'sheen' on the sound, like a nicely mastered track.
Instrument separation is 100x better on the Sennheisers. When a guitar is panned to the right, it's on the right outside of your head. With the Denons the right-panned guitar is sitting ON the headphone, as opposed to outside it.
Treble is better on the Sennheisers, there is little sibilance and harshness. You can really crank these and not feel fatigue. Cranking the Denons will kick and punch you, but if the music is good, you may want that.
Overall, I'd give the slight edge to the Denons for listening, and the Sennheisers for mixing. If there is an amp that makes the Sennheisers "jump", I would really like to know. As of now, I use both headphones with my Focusrite Saffire Pro 14 audio interface.
