Keep in mind that according to Gene DellaSalla that even if the internal amps of AVR are not being used, and still turned on, they introduce distortion into the preamp section as volume is increased, so the internal amps must be shut off like in the high-end Denons. So adding an external multi-channel amp to an AVR is more of transition to having a separate pre/pro and power amp, so factor this long term transition into your budget.For sure, without question its really disappointing that the pre-outs on pretty much all receivers apparently are not well implemented. That's a convincing explanation and it does reinforce that adding the external amp is not the final answer, even if its a good upgrade in the meantime.
Since I’m not a serious videophile owning a somewhat newer high end AVR is a better choice for me. My 10 yr old Yamaha RXV2700 140 wpc 7.1 avr is a good compromise for me. Stepping up to something a bit newer with high power may be a good alternative to separates.
It also provides a good explanation for my original question of why this isn't more common, I'm very appreciative for the info here.