We use a DI box supplied to us by Kemper. It looks like a tiny version of the lunch pail Kemper. It works well and seems to only color the sound slightly.
Merged profiling seems to be the most accurate, relative to the studio profiles, at higher gain settings (our High, VHi, and Max profiles). We rarely have trouble getting a good merged profile under these conditions.
Merged profiling can be rather frustrating at low and medium gain levels (our Cln, Low and Med profiles). The Kemper sometimes simply won't capture an accurate merged profile at low or medium gain levels, and you end up with something which doesn't sound at all like the studio profile. Sometimes it works great. Sometimes it doesn't. It's hard to predict when it'll be a problem and when it won't.
This is why we still include studio profiles in all of our packs, as they are the most accurate in capturing the sound of the amp.
One way we've found to get merged profiles to work at low and medium gain settings is to reduce treble and presence a bit on the amp. Sometimes this does the trick. Unfortunately, that means the merged and studio profiles are more different sounding than they would have otherwise been. We'll then adjust the merged profile after the fact, but there will still be some differences.
-Jimi