Best multi-purpose subwoofer

ghulamr,

     It seems like you've decided to go with a single sub in your system.  You'll still be able to get good bass response with a single sub if you're willing to accept just having a single bass sweet spot at your listening seat.
     I'd suggest looking at Monoprice's sub offerings and you could buy 2, or even 4, for about your $600 budget.  These will not go as deep as the Earthquake or NHT but I think having 2 or more 12" subs will give better results than a single 15" sub. 
     Remember, all of these subs are run in mono  and you'll get the dual benefits of cumulative bass output(you'll perceive the bass as more powerful but will not be able to pinpoint where it's coming from) and the reduction of room modes that begins when you have 2 or more subs in a given room (bass will sound smoother and more natural).  Also, neither sub is being over-driven.
     I know in any room, 2 subs will provide better bass performance than 1 sub, 3 better than 2 and 4 will provide exceptional bass response.  I can advise you on proper positioning when using any quantity of subs from 1 to 4 and in between.

    For the optimum positioning of 1 sub in your room, I'd recommend placing the sub at your listening position and play some music with good and repetitive bass.   Then, starting at the front right corner of your room,  slowly walk along the perimeter of your room counter-clockwise until the bass sounds best to you.  When you find this exact spot, place your sub at this position.
     Next, sit at your listening seat and replay the same music.  If the bass sounds just as good to you, then you've successfully positioned your sub.  If it doesn't sound just as good, you can either make small adjustments to the sub's position through trial and error until it does sound good to you or start the process all over again from the beginning. 
     
   Tim

millercarbon:

" Like, we all talk about this stuff with regard to bass, when in reality its not only bass but all frequencies. With bass its in your face obvious. But its across the board."

Hello millercarbon,

     From your last post, I can tell you have a good understanding of all this stuff.  
     I agree with your comment that all frequencies are important in a room for good sound reproduction.  I've reached the conclusion that, since bass frequencies behave differently than midrange and treble frequencies in any room mainly due to the large differences in the length of their sound waves, it's best to treat them separately.
     I've learned to get the bass response functioning properly first since it's the most difficult to get it sounding good in most rooms due to the very long lengths of low frequency sound waves.  I think you agree with me that a precisely positioned 4 sub distributed bass array is the best method of achieving this.  There are no mics, room correction software or hardware, parametric equalization and room treatments required.  The complete Audio Kinesis Swarm or Debra bass systems work very well, come with everything needed and are relatively inexpensive but any 4 subs will work as well or better as long as they're positioned properly, although they'll cost more.
     Once the bass is sounding good, the next step is to get the remaining midrange and treble frequencies sounding good which involve different room considerations due to the much shorter sound wave lengths involved.  These frequencies also reflect off room boundaries but colliding midrange and treble sound waves are so numerous they're normally perceived as an 'airy' quality.  The most important factor for good stereo imaging is that the direct, non-reflected sound waves from both the l + r speaker reaches the ears first before any reflected sound waves do.  Room treatments, that absorb or redirect first reflections, are proven methods to ensure this happens at the selected listening position. The later reflected midrange and treble sound waves arrive at the listening position, the better.
     But I'll refrain from further discussing the proper speaker and room configurations for good midrange and treble response at the listening position since most reading this are likely already knowledgeable about them.

Tim

You old optimist, you!

millercarbon,

     Just callin' em like I see'em.  I'm just enlightening guhlamr to some pertinent truths.  It's his money, system and choice.  Besides, I'm almost certain he'll be happier with multiple 12" subs than a single 15" sub. 
     Everybody also knows I'm the chump responsible for guhlamr's happiness.  And let me tell you, millercarbon, it's a lot of gosh darn constant pressure!

I'm just hoping I don't blow a gasket,
Tim