how big is your space?
If your existing SS is working, now is the easiest time to try/compare tubes. Your efficient speakers make it easier to try low power, not just lower cost, but less heat, size, weight, and more placement options.
how much power? Based on prior tube amps, I tend to think around 30 wpc, however I am very pleasantly surprised by the combo of my smallest tube amp ever, Luxman SQ150N: 10 wpc/6 ohms, and least efficient speakers ever, wharfedale diamond 225’s, 87 db. Plenty volume for my office, which is why I asked how large a space you have. My friend drives 'not too efficient' speakers with 8 wpc monos, no subs, also 10 x 12 sized space.
you also have the advantage that you are sending the hard work off to the self-powered sub, so a low powered amp can make a lot of mids and highs easily.
new or used?
I would find something that truly appealed to me, used, returns accepted, ask questions, try, keep or return it.