I totally understand your concerns, I did have them myself some time back. A few months ago I moved away for music school, bringing with me a shitload of gear including a Marshall stack and the Kemper Powerhead.
I have played a lot of guitar since then; live gigs, but mostly in a steamy and sweaty practice room with two or three other equally sweaty guys. Long story short: after fighting it way too long all the tube amps have to go, because the Kemper is simply the superior piece of kit in every way!
I mostly use it with custom Zilla cabs strategically ordered to maximise volume and headroom (8 ohm and 100db speakers), and when I try to turn the volume past five I start to get complaints and have to turn down again... Haha! If you get the cab rewired and it still won't play loud enough, you should borrow another cab with max 8 ohm and 100db speakers like V30s or G12H30s. I did actually notice a larger difference between 97/100db speakers than 8/16 ohm.
Just my two cents