It's actually surprising (to many people) just how little amplifier power is actually being used most of the time. If you look at most speaker sensitivity ratings, you'll see that they're somewhere in the range of 85-90dB or so. Some speakers are less efficient than that, some are more efficient, but most speakers tend to be somewhere in that range.
As I mentioned, that's when 1 Watt is being supplied and the measurement is being taken from 1 meter away. It's also supposedly in an anechoic environment. When you bring a speaker into any real room, you get reinforcement of the sound - ie. sound reflecting off of the walls, ceiling, floor and other surfaces - all of which combines to make the output of the speaker somewhat louder than what you would hear in an anechoic chamber. You'll generally get somewhere in the range of 3-6dB more output in an actual room.
With subwoofer, the "room gain" from that reflected sound can be significantly more since bass waves reflect so well and are so large and long. You might get 6-9dB more output than what you'd measure in an anechoic or quasi-anechoic environment.
Every time the output of any speaker goes up by 10dB, it requires 10 times the amount of amplifier power! So you can understand how the maximum amplifier power available can be very quickly reached!
As an example, when Audioholics measured the SVSound PB13-Ultra DSP subwoofer, they measured that in a quasi-anechoic environment from 2 meters away. The PB13-Ultra DSP was able to hit 115dB output levels in this setting, and that was the limit due to the amp, which can deliver a rated 1000 Watts of power. These are not exact figures, but good enough to get a rough idea. At 105dB output, the PB13-Ultra DSP would have been using about 100 Watts. At 95dB output, it's only using 10 Watts. And at 85dB output - which is reference level for the "average" loudness in a movie - the PB13-Ultra DSP is only using 1 Watt of power from the amp!
Most of the time, people don't listen at "reference" volume. Remember, the PB13-Ultra was out in a wide open field, with no surfaces around it except for the ground. Nothing to reinforce its sound. And the microphone was 2 meters (about 7 feet) away! In THAT setting, it's using a mere 1 Watt of power to deliver 85dB of output, which most people consider fairly loud! Most people have the volume in their homes down at an average level of only 75dB or so. For that little output, the PB13-Ultra DSP - in a wide open field - only uses 0.1 Watts!
Stick the subwoofer in a room and you'll get around 6dB more output from all of the room reflections and reinforcement. Suddenly, getting 85dB reference output levels, even sitting 12 feet away or something, uses barely 1 Watt of power! So it's not so crazy to state the specs and the frequency response using what seems like such a tiny amount of power. But you should also remember just how quickly the power demands increase as the volume level demands increase. You have to double the amount of power for every 3dB extra output. And use 10 times the power for every 10dB increase. If you've got something like a 300 Watt amp (like the one found in the Rythmik FV12, which is a great little $500 subwoofer), then if it produces 80dB using 1 Watt, it'll produce 90dB using 10 Watts, 100dB using 100 Watts and 103dB using 200 Watts and around 104-105dB using 300 Watts. Now you've maxed out its amp, but you're no where near the 115dB peaks that THX and movie theater reference subs are required to be able to hit.
So basically, using only 1 Watt to state the specs isn't crazy because the output using only 1 Watt can actually be fairly loud. But demands for power go WAY up and they go up quickly when you increase the volume
Anywho, a 4000 cubic foot room is definitely classified as "large". If you watch more movies than music, I would suggest to you that output and deep bass extension are important. I wouldn't really suggest a sealed subwoofer in that price range for that size of room and for that material. That said, you still want nice, tight, clean, accurate bass for both movies and music. Most ported subs won't be quite as tight as a sealed sub, but that does not mean they have to be "flabby" in any way - not by a long shot! I would put a ported sub like the SVS PB12-Plus DSP or its cylinder sibling PC12-Plus DSP up against many sealed subs in its price class, and the ported Plus subs would give up very, very little in the way of tightness or accuracy, while kicking the pants off of those other subs in terms of deep bass output and sheer loudness. So don't get too hung up about the "delay" and tightness stuff that I was talking about. I was simply saying that if you're using a pair of subs, it doesn't make sense to use two different subs. You won't so much get the strengths of both subs. Instead, you'll get the "weakest" link, which does NOT mean poor performance, it just means that one can't play as low as the other, so you're not "doubling up" in any way there, and one is a little bit less tight, meaning the tighter sub doesn't bring the less tight one up, the less tight one "drags" the tighter one down. Just not the best use of two subs. But do NOT take that to mean that a ported sub cannot sound tight and clean and accurate. It's a matter of SMALL degrees of difference
For a pair of subs in the $1300 range where you want great music performance, but play more movies than music, and you have a large 4000 cubic foot space to fill, my first recommendation goes straight to a pair of HSU Research VTF-3 MK4. Now, a pair of those big subs would be over your $1300 mark by a bit, especially after you factor in shipping. I'm not sure if your budget needs to account for shipping as well. The VTF-3 MK4 themselves are $700 each and shipping is $100 on each. I don't know where you're located, but HSU does allow you to pick up your subs if you're close to them in California and want to save the shipping cost.
A very strong alternative is the Outlaw Audio LFM-1 EX at $650 each and similar shipping costs. Dr. Hsu is involved in the design of Outlaw's subs and they perform rather similarly as a result. Audioholics did a full review of the LFM-1 EX, so that would be a very good thing to read if you want to get a nice idea of what the LFM-1 EX or VTF-3 MK4 can deliver. High marks
Those would be my picks for your situation. I think you'd be quite pleased with both the music and movie performance, especially at the price point. Even SVSound's least expensive NSD subs would be over your budget for a pair. A lone SVSound Plus model sub might do well though - it'd be worth consideration if you think a lone sub might work as would the ported 15" subs from Rythmik - although you should definitely check on availability of Rythmik's subs as their suppliers in Asia have been slowed by the unfortunate natural disasters over there. The Rythmik FV15HP is pretty much neck and neck with the SVSound PB12-Plus DSP in terms of performance - and is almost identical in cost once shipping is included. Great, great subs for that $1300-ish range.
But for a pair, I don't think you'll do better than the HSU VTF-3 MK4 or Outlaw LFM-1 EX. I think they fit the bill extremely well for your situation
