Well, here is a a problem that occur often when you change from one FRFR box for a new one. You use you tweeked profiles (with your older cab) with your new cab and boom too much of bass not enough of mids etc. The best way is to compare the new cab with a good studio monitor, not with your older cab which was most probably not especially flat itself. Or you can also choose somme untweeked profiles (from M Britt for example) which has the reputation to be very thru.
Maybe you're right and the C+ kit is too much bassy, but maybe it was your older cab that lack of bass and if it's the case then you will have to tweek all your profiles and not apply a general EQ to this cab.
Nope. I'm playing over studio monitors at home and dial in my sounds that way. When I'm in the rehearsal room I use the master EQ to make the sound fit to the guitar cabinet that is used.
I find the different rig presets inside the kemper THAT good that I tweak the sound only minimal. Maybe a bit more/less definition or presence that's all. I didnt' touch the bass at all.
I agree that a FRFR cab should be as neutral as a studio montor.
But to be honest - it's basically not a studio montior. It's still a guitar cabiniet with only a full reponse speaker built into it. So I think you still have to play with the master EQ settings...
By the way: The DIY Kit sounds awesome once you dialed in your EQ settings...