Hi,
I'd quote from Moz: http://moz.com/learn/seo/domain-authority
"How do I influence this metric?
Unlike other SEO metrics, Domain Authority is difficult to influence directly. It is made up of an aggregate of metrics (MozRank, MozTrust, link profile, and more) that each have an impact on this score. This was done intentionally; this metric is meant to approximate how competitive a given site is in Google.com. Since Google takes a lot of factors into account, a metric that tries to calculate it must incorporate a lot of factors, as well.
The best way to influence this metric is to improve your overall SEO. In particular, you should focus on your link profile—which influences MozRank and MozTrust—by getting more links from other well-linked-to pages."
So answering your question, Yes, you can improve the DA, while working on your SEO.
In general, in my belief, the 301 redirect could have influenced the score, but I would rather check the rankings and organic evolution of the website after the redirect instead of checking only the DA. If the organic results improved, I wouldn't worry to much about the DA itself. (maybe it is just only a temporary glitch until Moz recalculates it's value based on the new information they get).
Keszi