Hi Dave,
What you have noticed is to be expected. A guitar cabinet contributes roughly 70% of the over-all tonal signature, in an amp-cabinet system. In other words, if you play various tube guitar heads through a single, specific cabinet, you will get a lot of "sameness", since the cabinet imparts so much of its character upon the resulting tone. Conversely, if you play a single, specific amp through various and sundry guitar cabinets, you will get widely different sounds.
The same exact phenomena occurs when you connect a KPA to a traditional guitar cabinet. It may still sound great, but you lose a lot of the individual character and sonic uniqueness of specific studio amp/cabinet profiles. As you are aware, this is exactly where a FRFR-type monitor shines. It will faithfully reproduce the KPA profile of the reference amp/mic/cabinet system. So, a Fender Bassman sounds like a Fender Bassman, a Morgan AC20 sounds like a Morgan AC20, and a Marshal 1959SLP through a 4x12 Greenback sounds like (etc., etc., you get the point.)
My next research topic will be the AXE FX III . It seems Fractal has made some substantial improvements on their clean amp sounds since the last time I tried one of those. Though I am very pleased with my Kemper , I would really like to play an AXE FX III and sample the cleans . I am already sold on the Overdrive amp sounds. I tried an AX FX about 7 years ago. I liked everything about it except the clean amp sounds. And on that note, that is why I gave up on Channel Switching amps years ago. In the late 80's early 90's I had a hell of rack rig. Egnater IE4 preamp (The red One) and a mesa 2:90 power amp. That rig got as close as I ever have been to having really decent cleans and killer overdrive Mid to High gain amp all under one Hood. The Kemper has proven to be better than that rig was . I am also curious about the Synergy stuff. All these last three years some killer amp technology , Cab sims, IR's has surfaced with some very impressive break throughs . A guitar buddy of mine has an interesting rig , He has a 2015 Friedman BE 100 . He called me the other day bragging about his addition of a Syngery Module with DLX . Claims it gave his BE 100 a much better clean channel . For much of the last 20 years I have been very used to hearing what ever amp rig I use mic'd up and blowing at me through a decent floor monitor . . . just as much , or more so, than the amp it's self. So the transition to the Kemper was fairly easy for me to get used to. To have an amp rig that only takes up three or four rack spaces with FX and all and one or two Light weight cabs and it sound 99% like the real amps is my Goal. I am sooooo close to that with the Kemper !