I just finished building a Greenboy fEarful "12/6/1 cube" cabinet for my Kemper and thought I'd share my results. My goal was to have a FRFR cabinet that was suitable for guitar, but could also handle enough low end for bass at a real gig. I think this cabinet meets these goals pretty well. If you aren't familiar with fEarful, they are designed to handle super low-end very cleanly and are favored a lot by bassists who use low B and even low F# tunings. Don't let the single 12" fool you. It's rated for 533W (conservatively compared to commercial offerings). If you play bass or low-tuned guitars, it's definitely worth checking out. It's brown-note ready. 
I built mine as a 3-way with the "True Xover" crossover designed for flatter frequency response than the older 3-way system geared towards bass only. I suspect though that even the 2-way with the simpler crossover might even work well for guitars since it rolls off at around 5KHz IIRC. I just decided to go full range for more versatility. My build cost around $650. They can also be purchased from authorized builders for around 2x that. A <5KHz 2-way would also be significantly cheaper. The "cube" size fits the rack head, but there is also a taller narrower version that would look great under a toaster.
