I was about to suggest a sub, but I think the size of the room plus Erik's wise general advice leads first to some room analysis.
That said, what I see is a pretty hard to drive bookshelf with a tube amp and no sub. Then again, this is a very beefy, well made tube amp. So, I just am not sure whether you'd need or want a sub.
If I were in your situation, I'd measure the room with the speakers and listening position in as many practical configurations as possible and see how the bass measured and sounded. In my own case, the best result was a combination of positioning, room treatments, subs, and some partial blocking of the ports on the main speakers. That yielded a near perfect bass response from 20-300 hz.