Have run single driver loudspeakers for 17 years but admit that there is no perfect loudspeaker (although my JBL 708P's, active monitors, were the best I've ever heard). Single drivers can't quite cover the modernly defined "full" acoustic range (20-20,000 Hz) and larger drivers "beam" at higher frequencies (those shown by FullRangeMan start beaming around 2,000 Hz). Of course single driver loudspeakers lack nasty cross-overs, differing voicing between drivers, and different source locations for multiple drivers for improved imaging. My loudspeakers are no longer available but suit my tastes well. O.B. have vastly different presentation than monitors (wall wide images, in FullRangeMan's case coming from the floor) and lack chest pounding deep bass.
Selling your current power amp and loudspeakers would allow for active monitors to fit in. (If running a multiple channel system, you're right about mixing active and passive speakers.)
Suggest ditching/moving/lowering the gear/rack between loudspeakers as they block the sound stage (sound cannot occupy the same space as solid objects). That's what I like about my simple system that uses tiny Temple Audio mono-blocks and one small box server/DAC.
