High Efficiency

Looking to pair 2a3 DHT power amp rated at 10 watts with some high efficiency speakers. At what decibel levels would the speakers have to be and what is a good used speaker choice? Klipsch Heresy11?
Any Klipsch ought to work. What they have to be depends on how loud you like it. For a rule of thumb the power needs to double for each three decibel increase in volume. For example, my Sansui SP-X7000 speakers have a rated efficiency of 97db and power handling of 130 Watts maximum. This means they'll reach meltdown at just over 128 Watts producing 118db. However, my solid state amp is rated at 65 Watts not to exceed .025% THD, total harmonic distortion. This means that once I exceed their 65W rating at 115db, distortion may rise until the amp melts down. What makes your question difficult to answer is how your amp is rated and how loud you want your sound. Your ten Watts might still sound pretty good distorted and might be rated that way, say at 5% distortion. Speakers rated at least 95db ought to keep you happy in a domestic environment.
Depending on your budget, any one of the Devore line of loudspeakers would be a great choice. They thrive on low power tubes.