135 wpc solid state shouldn’t be underpowered for a harbeth hl5 unless the room is very large and/or you listen hella loud
that been said, i drive my harbeths with any of the amps below
-- audio research reference 110
-- audio research v70
-- primaluna dialogue prem hp
-- hegel h390
-- odyssey stratos
-- audiosector patek se
-- at times even a naim xs2 with teddy cap
harbeths are tolerant of many good quality amps, choice of amp depends on musical taste largely
that been said, i drive my harbeths with any of the amps below
-- audio research reference 110
-- audio research v70
-- primaluna dialogue prem hp
-- hegel h390
-- odyssey stratos
-- audiosector patek se
-- at times even a naim xs2 with teddy cap
harbeths are tolerant of many good quality amps, choice of amp depends on musical taste largely