I've been thinking about making this tincture as well. As far as I know, tinctures get stronger the longer you let them soak. So rather than soaking it for two weeks, shaking it every day, you could do it for longer. A two month tincture period would be pretty strong. Also, according to Uniquity on the natural healing forum it's important to use 100 proof alcohol and preferably grain alcohol, although she said I could take 190 proof grape alcohol and add distilled water to it to make it 100 proof. Another words, I don't think you want stronger alcohol to make the tincture more potent. It has more to do with the amount of time that you wait.
Anyhow, make sure to check out my recent link here over to uniquitys forum about ashwaghanda, so you can decide where to order it from.