Low level is better.
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I have always preferred the high level myself. I am currently using a couple REL subs, and that is what they recommend. You are getting the exact same signal at the exact same time as your speakers. Now if you are using this in a HT set up, I would go with the low level. Get yourself some cheap speaker wire and give it a try. Then if you like it, step up to some nicer cables.
For a music only system, go with the high level inputs. I have tried both ways with my Martin Logan Depth, and the high level is a noticeable improvment. It integrates your speakers and sub better. And the sub does seem to keep up with the speakers much more. I remember several years ago reading on the REL site their logic behind it. And when I finally bought a sub and tried it, they were right.
High level means your amps are putting out the signal for under 20hz....IMHO and IME all amps sound better when not driven to the lowest. I would recommend using low level inputs and dumping the lowest freq. off from your full range amps.
Some subs have a line level XOver...I had very good results with Passive Line Level Xover, say from 50hz up!
You will win a lot of detail and dynamics on the mids and higher freq.
IMHO that is the most important feature of a subwoofer, leaving the lowest Freq. to a speialized sytem (the sub) in order to allow your full range system to breath better on top!
I've heard that using high level inputs bypasses a stage of gain so the sub's amp runs cleaner. This is of course assuming that the amp in the sub is of lesser quality than the main amp, which is often the case. I have a PS Audio GCC amp with remote volume control feeding my subs- I love after all these years of no bass control being able to dial in remotely how much bass I want per song/LP.
I guess it's obvious by now you're not going to get a definitive answer and certainly not a consensus. With my speakers and sub, an HSU, the high level inputs make for a better integration between speakers and sub and that setup is what was recommended by the manufacturer, even though they make provision for using either high or low level inputs.
When you use high level inputs, itdoes not cause your power amp to work any hardr, because the input impedance of the sub is so high, it does not draw much current/power from the power amp. It is as if you had all the power of the power amp in a preamp. Talk about headroom!! (and I do no tknow which would sound better).
I have two subwoofers, one in the living room for an LP-sourced stereo, and one in the 7.1 home theater rig.
For the home theater rig, I use the low level input and a fairly low crossover point handled by the surround pre/pro.
In the 2-channel system, however, I use high level inputs. I've tried it both ways, and it's not even close. Using low level input, the whole system sounded lifeless and uninvolving, and it seemed like I had to turn the subwoofer way up just to hear it where it should be.
I also noticed a second improvement when I upgraded the speaker cables from the power amp to the sub. I went to some of those garden-hose sized PS Audio XTreme Preludes from an Audio Advisor sale ($50 for 6' pair), and it made the entire image and clarity bloom.
I first heard of this reading Pierre Sprey's free audio tips on his Mapleshade website, http://www.mapleshaderecords.com/audioproducts/freeaudiotips.php, where he says,
For seamless subwoofer sound, use only the speaker cable input, not the RCA input. In addition, connect the two main speakers directly to the main amp output, not to the subwoofer's output. Always fire the subwoofer driver left or right, not at you or down into the floor. Set the crossover at the lowest possible frequency that doesn't leave a bass gap. You'll be amazed at the overall transparency you gain.When I first read that, it defied all logic, but I tried it both ways, and at least for 2-channel stereo for music, with the Mirage subwoofer's I've used (LF-100 and LF-150), it definitely works better.
Pierre brings up a separate issue in his tip. Running the mains full range and using the sub as out of the way as possible keeps the bulk of your music uneffected by the sub as well as not having to go through the extra degradation of the crossover. The only down side to this is if you want to prioritize unburdening your mains with low frequencies by crossing them over higher. Crossing over the mains definitely has advantages, especially when playing loud, but the disadvantages in terms of transparancy are significant.