You'll feel both focused and relaxed. We can't tell you what drugs to take to get high your question is really close to that...and furthermore we can't tell you exactly what you will feel like because everybody reacts differently to drugs to one degree or another. But chances are you'll feel relaxed but still awake due to the Dexedrine. You may not remember all of the experience especially since you don't have a tolerance to benzos, amnesia is common in people without tolerance. You could get impulsive and do something stupid. It really is impossible to tell you exactly how you'll feel.
Just as an aside, a speedball generally refers to a stimulant + an opiate. A "true" speedball is morphine + cocaine (or heroin + cocaine) but that's just semantics.