About the ReSound Key
Key is the latest "essential category" hearing aid family from Danish hearing aid manufacturer ReSound. In terms of core technology, Key can be considered a more budget-friendly version of LiNX Quattro, ReSound's previous-generation premium product line, which was recently superceded by ReSound One. Notably, Key lacks many of the advanced binaural features found in LiNX Quattro, such as Binaural Directionality, Binaural Environmental Optimizer, and Spatial Sense. The directional microphone features are also less advanced.
One thing to note about Key is that the lowest technology levels (1 and 2) do not include airect audio streaming via iOS or Android. So, if you're considering purchasing the Key aids, and audio streaming is important to you, you'll probably want to look at either the 3 or 4 level. All levels are compatible with ReSound's popular Smart 3D app.