As they say, if this don't light your fire, your wood's wet. Crank up your volume and listen to I L-O-V-E U. 6 amazingly skilled vocalists came together to form the a capella group Take 6 in the 80s (yes, there was such a time). There are only vocal sounds on this song - no instruments at all. Listen to each part and notice how skilled and difficult each separate thing is to do. Harmonies, lead, licks, fills, percussion, performance conception, musical energy - all amazing and ground breaking at the time. This is HARD music and it doesn't suffer from being all vocals, the voice being a comparatively imprecise instrument.