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.

I have 2 JL f113's. When I first got them I played around with the difference between one on, and then both on. The bass is certainly much fuller with two subwoofers. You can get away with one, but two are better. You'll be able to achieve a much flatter response in the low-end with two.

I also have two JL Audio F113's. I have them in a 20 X 15
room. I did start with one in the room and when I added the
second later that day after listening to the single F113
the bass response was even smoother. The room loaded better
on low end material and seemed to tighten up. It made me a
believer. It depends on the size of your room and budget.
My opinion is two subs placed properly will sound better
than one.

Even Fathoms or Gothams that have plenty of power for a certain room will greatly benefit from a second, with better bass loading, but, also, and more important to me, is much better imaging without shift. It's just wrong what is said about not being able to locate LF sound sources. Even with a steep cutoff at 35 hertz I can easily locate a single sub playing in my room. My 6 year old said "what is that box making noise" when he heard the system with a sub. I asked "what noise" and he said "brrrmm brrrmm". Two of them close to the speakers is the ticket.

I agreed with Kiddman and others, 2 is better than one, from my observations in my system I was always able to locate the single sub when I had one no matter where I placed it. Then I bought another and put them close to mains and bass is smoother and very hard to pinpoint the bass location.

Thanks for all of this valuable information everybody. I have decided to get another subwoofer. Now I have a new problem. For the life of me I cannot locate another subwoofer to match the one I currently have. Velodyne stopped making the 1500r spl years ago and I cannot locate one through this site, eBay, craigslist or any where else. Can I set up a similar in specs but different model, or must they match for sonic reasons? I have given myself a headache!

Sell yours and get two DD12 or15 instead. These were the best velodynes made.