Hi there, I want to buy a pair of over-head headphones. Im not decided between Sennheiser HD 280 Pro, Sennheiser HD 497 and Sennheiser HD 515. Allow me to list some specifications for these headphones: Sennheiser HD 280 Pro: Sensitivity: 113 dB Impedance: 64 Ohm Frequency Response: 8 to 25,000 Sennheiser HD 497 Sensitivity: 112 dB Impedance: 32 Ohms Frequency Response: 24 to 22,000 Sennheiser HD 515 Sensitivity: 104 dB Impedance: 120 ohms Frequency Response: 14 to 26,000 Now, my questions: 1) Is it a big difference between 113 dB (Sennheiser HD 280 Pro) and 104 dB (Sennheiser HD 515)? 104 dB means I lose A LOT of dynamics? 2) Do I need a headphones amplifier for Sennheiser HD 515 (120 ohms)? That will be an inconvenience, because I want to use it with my iRiver. 3) Is it a big difference between 24 to 22,000 Hz (Sennheiser HD 497) and 8 to 25,000 Hz (Sennheiser HD 280 Pro)? Do I lose a lot of bass? Can I hear 8-25 Hz? Thank you, in advance. Daniel PS Any suggestions for other headphones: Sony, Koss, Grado, etc.