For me, 400mg of caffeine would be too much, but each to their own. It should be quite safe to combine but that wasn't really the point I was getting at - I was just pointing out it's a bit silly to have predetermined that you will need - or want - to combine anything with it until you've actually tried it. A number of people do find the stimulation to be lacking, but many others don't. I've actually never combined 6-APDB with a stimulant myself and enjoy it very much on its own for itself. It's a very nice drug and I've yet to doze off whilst under the influence of it.
Personally, I'd not choose to use caffeine as a source of primary stimulation at all as it's so damnably jittery and just feels unpleasant taken at high doses. If you happen to enjoy high doses of caffeine, I don't see why it would be a problem per se. It's ultimately not really any different from adding speed to it though - you're still assuming a combo of some sort is a requirement rather than a potential variation. I really would recommend taking 6-APDB by itself at first - see what it does itself, see if you feel you really need anything with it. I don't feel I need anything with it but (after quite a number of experiences with it alone and in combination with other drugs (mainly psychedelics)) I do quite fancy trying it with a straight stimulant, but I don't feel it needs the addition of a stimulant anymore than it needs the addition of a psychedelic - they're just different experiences.