Hi Sugarlou,
I also struggled a long time with the kemper and a FRFR (in my case first single, later dual yamaha dxr 12). From my experience (for what it is worth ...) you can't get a FRFR system sound the way a 1x12 or a 2x12 cab sounds. I think both solutions have their strenghts and weaknesses. It seems logical that a FRFR will never sound exactly like a 1 or 2x12 because these are totally different soundproducing devices:
Amp: FRFR flat response with no respons between amp and speaker, crossover design.
Speakers: FRFR does have tweeters, the speakers of a cab are not picked to have a flat response but are tailored to sound good with guitar. A FRFR is linear in respons with volume while a guitar speaker is not (compresses somewhat at higher volumes). Sometimes in a 2x12 cab two different speakers are used to balnce the overall sound even more.
Cabinet design: FRFR is made not to resonate and has a bass port tailored to a full range response , a good wooden cabinet is made to resonate and has a specific 'bass port' (open (in all its varieties) or closed back).
All these differences are leading I think towards my experiences with these speakers:
FRFR has advantages at lower volumes (that is beneath the volume of a hard hitting acoustic drummer) At lower volumes we like to hear some more bass and treble (our ears are most sensitive to middle frequencies, but with higher volumes sensitivity is more even across the frequencies). The Kemper is extremely good at maintaining control over the volume and the gain. Escpecially when you play at lower volumes and you play with other soundsources that have balanced volume output (like keyboards or electric drums) this is a great advantage. With a guitar cab you can also easily isolate your sound somewhat too much from the keyboard player. Tube amps with guitar cabs at lower volumes often sound somewhat thin and 1-dimensional.
Guitar Amps have advantages at higher volumes (f.e. in a band with hard hitting acoustic drums): the evenness and richness of harmonics (the '3d quality' or 'amp in the room sound') of a quality amp and cab on it's volume where it is designed for can not be rivaled by a kemper and FRFR imho (but maybe with a kemper and Solid state or tube-amp and cab , i don't know, i dont have experience with that). Another advantage is when you play with two guitar players in a rock band two cab's are much more easy to be differenciated from each other as compared to FRFR on these volumes. Lots of tube amps these days have limited wattages so sound on their best (with crancked poweramp tubes) at bandvolumes which are 'reasonable' (but that's a very subjective term).
Of course this is totally written form the pesepctive of the guitar player. Another point considering is your audience 
The kemper sends out unrivaled signal to the PA (no problems with miking or leak from other soundsources) and when you adress your public through a pa the Kemper is better than a miked amp, well at least much easier and consitant to set up. On the other hand when you gig the 'old fashioned way' (with guitars through the cabs and vocals through a pa) this can be great too (or even better imho,you give the audience some of your superb cab-sounds 
).
Another point is off course you have good and bad constructed FRFR as well as guitar cabs, and the issue of 'breaking in' speakers, and of course everbody bring sin their own ears and experience. If you play 20 years in a rockband with your tube combo and are totally in that sound, it really is a dfferent story when you play on a kemperFRFR. But when you practice at home with a line6 something or a jamvox, picking up a kemper will be jawdropping.
This just to share my experience. No way i want to criticise the kemper, it sounds very very good, the lag is not noticable, the effects are great. Especially when recording it is unsurpassed and the community and support is great as well, the free profiles and lots of great commercial profiles.
