sound pressure level. 90dB/mW. 111dB/mW. Devices with a higher sound pressure level are generally louder when supplied with any given audio source. has a neodymium magnet. Bose QuietComfort 35 II. Koss Porta Pro Wireless. Devices with neodymium magnets are lighter and more powerful than those which use ferrite magnets.
I strongly suspect that the variability of reviews on the Koss Wireless PortaPro sound is a reflection of the Bluetooth protocol. The headphones support Bluetooth 4.2. With AptX, the sound is just fine. Without it - paired with a mobile iOS device, there is a noticeable degradation of sound. Worse soundstage, slightly muddy reproduction overall.