Well, there is no preference in theobromine for cAMP effects that occur within the range of the DMT mind state. The brain activity that would normally be supressed by DMT would also be amplified by how theobromine intermediates in cAMP activity. This means that part of the DMT effects would be amplified and another part of it would be reduced. At least, that's my speculation.
I think THH is something that will make the balance tip to the other side, depending on the dosage. This would have to do with the increased amounts of DMT in the blood and the increased amounts in the brain itself as well.
On this forum there where speculations on wether the visual effects of DMT would be caused by it's serotonergic effects or it's effects on other transmitters. One argument here is that there are other tryptamines such as 5-MeO-DMT that affect the serotonergic system like DMT, but that lack visual activity.
At the same time 69ron once compared 5-MeO-DMT, DMT and bufotenin and it became clear that the most visual compound hast the longest lasting effect while the least visual compound has the shortest lasting effect. It's also a fact that 5-MeO-DMT becomes more visual when it's taken in combination with MAOI's and it becomes longer lasting.
I think that how long a molecule is active within the brain itself, may have something to do with this. This is ofcourse a pure speculation, but i think that 5-MeO-DMT has a shorter lasting effect on each receptor, that once it activated a receptor, it's removed by the brains mechanisms quickly. So if you block this mechanisms within the brain, not just the stomach, it will make any compound more visual.