What does a subwoofer solve?

Some manufactures of exotically large speaker systems and even some of those who include self powered subwoofers within their speakers often display them with outboard subwoofers. The location of the speakers optimized for the listening position is quite often in a rooms null which can choke the low frequency response. Adding outboard subwoofers in the rooms modes is the solution. 

A host of individually unique room issues can easily be solved depending on the flexibility and level of control of low frequency processing which is not generally focused on by this community. 

@m-db "A host of individually unique room issues can easily be solved depending on the flexibility and level of control ..."

100% agree. And I've experienced it first-hand with a DBA. Getting this right brings a night and day change. It is that big. 

I will say adding a sub done RIGHT makes a huge improvement in not just rap.  Getting to mix right is no easy.  I ended up with the JL Audio CR-1.  Not cheap.  But since the intro digital music a good external cross over is getting hard to find.  this did HT integration as well which was another Issue I had.  

I do have a BHK 250 recently upgraded from Krell KAV 2250.  It does have much more impact than the krell.  

I think you will find adding a sub especially if you can get the load of the mains with xover you will find it works so much better.  

May I ask what PS Audio could not get to work as advertised?

Some of my observations on using subs, in bullet points:

*It highly depends on your expectations, on how you listen to, and what you listen for.

*If you are looking for power delivery and just more (deeper or stronger), the sub will give that to you.

*When the sub amplifier is quite different from you main amplifier(s) driving your main speakers, on the long run this will cause a disconnect, and you will feel the strong urge of the upgrade bug without consciously knowing what drives this need.

*Adding a sub often compromises the harmonic structure of the music.If you listen for violin, piano, cello, then this might be a huge issue. For most people this goes unnoticed (or is not a priority at all), but if you are one of those (like me) that tone and timbe & violin means everything, then you might end up like me (never found a sub that integrated well - they did good things for certain material, but on the long run they failed at what I want most: natural tone).

*Be extremely careful with subs, just because we can, we always set them at higher levels than needed. There's the temptation, and I have not seen any of my friends deny extra bass..... result: unequivocal decline in hearing faculties. Of my longtime audio friends (been on the audio journey for 20-40 yrs), those without subs all have keen hearing, usually quite better when they were young, and the sub users had massive hearing decline.

*Even when you set the sub level (and resist temptation to boost bass), it will still increase listening fatigue. 

*Room / loudspeaker placement / amplifier matching is everything. Want better bass? Work on these, and it will give the most natural fully integrated sound. Adding a sub is a quick shortcut around these issues, and only you can tell whether it worked for you, or you need to take the long road.

Any choice you make, you will learn from it, and will enjoy it tremendously. This is all fun, and a great experience. Just be VERY mindful of your hearing faculties. You can undo any change to your stereo gear, but you can't undo damage done to your ears.

A subwoofer compensates for a human ear’s reduced sensitivity to the bass frequencies as demonstrated by the Fletcher & Munson curves.... as was the 'loudness' control of the older vintage amplifiers.