I think it would be hard to make valid suggestions without more information from the OP.
Does he want to stick with the tonal balance and sound that is similar to Harbeth? His other components like the DAC also lean towards a warmer musical presentation.
When he says low power, how low ... 3 watts or 10 watts?
Perhaps the OP has an amp in mind that would give forum readers a better reference point for speaker matching based out of their experiences. What tubes does it run output? 300B tubes or 6bq5s? Both sound very different.
Finally, what kind of music and at what volume does he listen to?
How small the your bedroom? One man’s small is 10 x 10, another 15 x 15 with high ceilings. The volume of the latter is almost twice that of the first and you would need more efficiency and/or power to fill the room.
i once owned a pair of the Harbeth C7s and despite what the specs said, they needed a fair bit of power to sing and worked better with my 100 WPC SS and than my 45WPC tube amp which is not what I expected. I liked the C7, based on a very favourable review without hearing them but they were not the sound for me, and buying used I moved them along at no loss.
Here's an idea. Buy your new low powered amp and try it with the Harbeths you own in your bedroom. If you think that works then you will know if a speaker of the sensitivity of the Harbeth will work fine. If it doesn't you can go with more efficiency.
Many readers use and will suggest Klipsch as a match for low powered amps. They are extremely efficient and are a good match, BUT, they don't sound anything like a Harbeth.