No such thing as a perfectly flat response in any listening conditions. Transducers of any kind always introduce humps in the curve somehwere. In your price range however Audio Technica have the ATHM40 "flat curve" headphones, but in my opinion they suffer from having closed backs. Makes the stage a bit small. But they still sound good. A slightly pricier pair of Sennheiser 518 or 558 cans are also an option. They are definitely a tiny bit bass heavy and the 518s tend to muddy up if you push too much low end through them, but the open back design makes them sound really big.