If you don’t listen to rock or demanding music which has lots of transients and burst of dynamics, or music with a lot of deep thumping bass lines, a low to moderate powered amp would be fine. Perhaps that is the reason you find the flea-powered Almarro amp too be sufficient for the 40.1. What you need is a tube amp that is able to bring the clarity and detail from recordings at low to moderate levels, music which doesn’t have a lot of dynamic swings or demanding bass.
A friend uses the Leben CS-600 on the 40.1 and the midrange to highs show higher levels of clarity and detail. But bass does not too deep or show the same level of punch as the solid state equivalents. It depends on what you want to achieve with your system, apart from musical preference. If you want holographic or 3-dimensional sort of presentation (I like this too), do try the more powerful tube designs if possible. I know most people will say power or current is not important for Harbeth but my experience tells me otherwise. I have used many solid state amps with the Harbeth SHL5+ throughout the years.
In summary, you would do fine with flea powered tube amps with the 40.1s or any Harbeth for that matter if the music isn't too demanding or you mainly listen to mellow stuff. For wider music genres and balance, an amp that is able to provide the dynamic swings at all volume levels would be more appropriate.
A friend uses the Leben CS-600 on the 40.1 and the midrange to highs show higher levels of clarity and detail. But bass does not too deep or show the same level of punch as the solid state equivalents. It depends on what you want to achieve with your system, apart from musical preference. If you want holographic or 3-dimensional sort of presentation (I like this too), do try the more powerful tube designs if possible. I know most people will say power or current is not important for Harbeth but my experience tells me otherwise. I have used many solid state amps with the Harbeth SHL5+ throughout the years.
In summary, you would do fine with flea powered tube amps with the 40.1s or any Harbeth for that matter if the music isn't too demanding or you mainly listen to mellow stuff. For wider music genres and balance, an amp that is able to provide the dynamic swings at all volume levels would be more appropriate.