The
whole thing with active monitors is that they have the amp built in and
you supply both power and a standard-level signal source to the
speaker. A plain speaker cabinet, though, would have just the two
wires running straight to the cones along which you would supply an
amplified signal source of arbitrary magnitude. In any case, your
soundcard is doing the outputting to a speaker and not the other way
around, so the speaker can't do anything to overload the circuit as
it's on the receiving end. A large passive speaker would,
however, short out (ish) the output stage of the soundcard, but since
the signal line is an ac coupled output you do no harm from that
either.