Yes, this is exactly why it works. It works for me in this same manner. I've always been too norepinephrine dominate and any drug, like Doxazosin, improves erection quality (in terms of maintaining them in my case). However the side effects are not tolerable for me.
I also tried Atomoxetine (Strattera) and another one I can't remember the name of for blocking norepinephrine, and they also improve erections. However each of these give terrible side effects as well (feel dead all day long, bags under the eyes and more).
In my case I have found either metformin or berberine work just as well as doxazosin with many other added benefits. They have improved erections as much or more than dox/nor blockers. I believe I have insulin resistance (have a little fatty liver, had high uric acid, never had good blood sugar control, etc) which is implicated in endothelial dysfunction and vasoconstriction. It seems these agents protects against endothelial dysfunction, lower blood pressure and relax the artery walls.
Metformin Improves Endothelial Function and Reduces Blood Pressure in Diabetic Spontaneously Hypertensive Rats Independent from Glycemia Control: Comparison to Vildagliptin
The Effects of Exenatide and Metformin on Endothelial Function in Newly Diagnosed Type 2 Diabetes Mellitus Patients: A Case–Control Study
My two cents for what it's worth
What sides you experiencing with Doxa?