It does smooth out the jitters from amphetamine side effects. I also tried a combo of xanax and adderall, it was the best adderall experience I ever had. Its like your heart rate never goes up, your body temp feels fine, and you get the pure head-rush of amps in your brain.
Now I am mentioning drugs that are slightly off topic, but somewhat similar. So I think you should be ok in your case even though I have not specifically tried valium and mdma under the same day. Mixing this upper/downer combo has not had a direct interaction against each other. It may only get dangerous if you mixed alcohol in the combo, I bet.