1. If a headphone spec includes very high frequencies above 23,000Hz, that's a good sign, and the headphone might sound clearer than a headphone with a 18,000Hz spec
2. The best bass is only achievable when the earpieces have an airtight seal
3. Home amps are more powerful than portable devices, and AC-powered amps produce more bass and tend to sound better with higher impedance headphones
With smartphones and portable music players, low-impedance headphones of 32 ohms or less are recommended. At home impedance is less of an issue, but Grell recommends higher-impedance models.
4. Larger drivers do a poor job reproducing high frequencies. Grell says the maximum driver size should be around 42mm (for full-size headphones); with larger drivers the surface of the diaphragm physically changes shape at high frequencies and produces irregular frequency response. Larger drivers can generate cleaner bass, but less accurate high frequencies.
5. "Trust your ears."