I didn't use an amp, I was at an electronics outlet with 46 different headphones on display all connected to a source with demo tracks hidden behind the display, I briefly listened to all 46 headphones mostly listening to the track "Last Nite" by The Strokes.
All those Sennheisers really sucked :( I think under the HD600 and HD25-1 II I Sennheiser is just another brand focusing on marketing, cheap sound and cheap plastic.
The Aviators were 'okay' sounding, yes they have decent balance and resolution and they are a cool fashion statement/design, in the end they sounded too detached/lifeless to me and I couldn't stand how vocals took a backseat in the performance, my favorite Skullcandy was most definitely 100% the Skullcandy "Uprock".
The low level Senns like the 238 are truly awful. Like the Bose QC3, they are unfixable even with EQ. OTOH, the PX-xxx-II's are usable as portables.