I wonder about the ambiant noise level in your listening room. It may be masking the details of your speakers. Headphones have close proximity to your ears and anywhere from a little to a lot of isolation. This can make it easier to hear details.
I've got excellent headphones, AKG K701s driven by a Woo Audio WA6 single-ended, class-A amp. You can see my speaker rig in the link below my name here. Anyway, I judge the health of both systems by how close they are to each other. They are very close in all respects, including detail. For the best listening on the speaker system I keep the ambiant noise level low, often turning off the AC, making sure the dishwasher is off, etc.
Dave