horns are so sensitive and revealing that they yield their input unforgivingly. Many blame the horns themselves, which is a significant misunderstanding. It's this simple: garbage in = garbage out.
You may not have a good equipment match for these speakers. Some tweaking experimentation may first be required in order to determine that. Quality interconnect cabling and AC cords are good places to start. Also speaker cables and AC line conditioning. Shelving, cones, footers, all the tweaks make a difference when driving horns. Different combinations work better than others: this is the phenomenon known as synergy. However due to the fact that the sonic signature was initially much more to your liking with the Onkyo, then perhaps the MH is not the better way to go. Tube buffer - I wouldn't waste that effort. Yes you must be willing to experiment extensively, but that's the fun.
Also consider your room as a part of the overall result.
My experineces with horns (incl Klipsch) have been most satisfying with solid state mosfet amplification. Accuphase works amazingly well for my setup and sonic preferences, which emphasise musicality over definition & detail.