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?