Small speakers are not necessarily more efficient. Klipschorns for example are amazingly efficient and don't lack bass at all.
Passive subs will not reduce the load on your receiver and will lack a crossover with controls to help blend with the speakers. If you get a sub, get a powered one unless you buy a matched set of passive sub and satellites.
Adding a powered sub will also dramatically increase your volume level. Bass is extremely power hungry. A 200-500 watt sub can do wonders.
Final note-the difference between 70 watts and 140 watts is barely audible, about 3db. To really notice a difference in power, you would need to go up to 200+ watts per channel.
Passive subs will not reduce the load on your receiver and will lack a crossover with controls to help blend with the speakers. If you get a sub, get a powered one unless you buy a matched set of passive sub and satellites.
Adding a powered sub will also dramatically increase your volume level. Bass is extremely power hungry. A 200-500 watt sub can do wonders.
Final note-the difference between 70 watts and 140 watts is barely audible, about 3db. To really notice a difference in power, you would need to go up to 200+ watts per channel.