truth is, many decent amps can work well with the baby harbeths
like all good speakers, they are fairly revealing of the source, so choose wisely
harbeths have good transparency but a warm overall tone, so if you like even more of that character then pick a tube amp - conversely, if you like more a see through, hear through, extended sound, then pick a good solid state unit
as harbeths have relatively relaxed treble presentation, even the lesser sharper sounding solid state units can sound pretty good... not as good as an ultra pure, more nuanced amp like a pass or ayre or hegel that will allow the speaker to do its magic in imaging and midrange palpability, but still pretty good