It's hard to say for sure. If you place two subs carefully, they will almost always (per the above responses) provide smoother response than one. However, if the subs are simply going to sit under the left and right main speakers, it's very unlikely that you'll be maximizing the response benefit from placement because that's not where they belong (in most cases).
OTOH, you absolutely will get more clean output capability in the bass, but it's not clear that you'll notice much difference. IIRC, the 1500 is a big, powerful sub and I'd think that one unit would probably be adequate for most musical program material, in most rooms, at most typical playback levels.
OTOH, you absolutely will get more clean output capability in the bass, but it's not clear that you'll notice much difference. IIRC, the 1500 is a big, powerful sub and I'd think that one unit would probably be adequate for most musical program material, in most rooms, at most typical playback levels.