There is no need to filter over 20khz, which your can't really hear. Most people can't really hear over ~12-16khz, above ~12khz you more or less don't hear it but sense it and creates "airiness or openness" in a recording. Tinniness actually occurs around 1-2khz and is the first harmonics from a cymbal. Silibance is usually what listeners complain about, which occurs at 6khz and "piercing" occurs around 10kz. Below is an interactive chart that's been around for decades that one can use to EQ a system. It's actually intended for recording engineers, but works for car audio guys too.
alexiy.nl