2 subwoofers?

I have always heard and it is my personal experience that 2 subs are better than 1. The reasons are: 1. It will load the room more accurately by reducing peaks and dips. 2. There's stereo information even down in the bass range that you lose with a single sub. It adds spaciousness and soundstage depth. 3. You certainly benefit from the additional power. It allows you to run both subs at a little lower level increasing your headroom.
I assume you will add some type of filter to roll bass out of your main speakers. This allows your mains to be cleaner and increased soundstage depth, etc.
The bottom line, IMO, 2 subs make for better sound. They are some additional reasons.

I have, in most cases, found that two subs are indeed better
than one.

Setup is a little more tricky, but overall, worth the hassle.

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.