The one thing that better components, cables and speakers do as you move up the line is bring out more low level bass detail. There is still no reason you can't improve a mid-fi system further by experimenting with interconnects, other speaker cables, and even power cords if your gear has IEC plugs.
The addition of a REL audiophile subwoofer will also bring out that low level detail, without changing the sound of the rest of your system that you enjoy. Check out the REL Stratus III or Storm III.