Parallel connections increase current flow if voltage is held constant. Series connections reduce current and increase maximum safe voltage. Two speakers connected in parallel will be louder with the same voltage input. Connected in series, they will handle higher voltage.If you want higher sensitivity and don't need higher power handling, go with a parallel connection. If you don't need higher sensitivity but do need a higher maximum SPL, go with the series connection. Either way, you'll want to make sure you're staying within the operating range of your amp.