Guidance

Adding a sub makes a huge difference!! If you tune it properly, it will totally disappear to the point you will ask yourself if it's turned on. It's something that I wish I did sooner on my system. It allowed me to move my speakers where they mid/high sound best, without loosing bass. With my tone controls set to 0 now, the amp isn't trying so hard to produce bass, so everything just sounds cleaner, fuller. 

You may also be siting in a null point, this will kill the sound. Play a song you know well, then walk around the room, does the bass get bigger/smaller as you walk around? How does it compare to your listening spot? Also make sure your speakers are in phase. Again a song with bass you know, turn the balance full left, listen, then full right, listen. Is it louder or softer at the extreme L/R? If so, they are out of phase. 

Of course moving the speakers around the room, can make a massive improvement! Also adding spikes to the speakers can enhance the bass.

I'd invest in an 'adequate' sub of your taste/budget just to 'dip a toe' and see what happens.  If it seems to moderate what you hear as a deficenceny, stay put and attend to the room....  If it doesn't have a level and xover controls, walk away; used ought to be fine....
Attend to the room....I've never really had the opportunity to delve into that other than simple cheap tricks, so there might be a chance at the big leap forward.

There's your homework... ;)  C'mon back when you've muddled about with the dragon, as there's really no strangers here....

Well, mostly...*L*

When you mentioned DarTZeel and said you didn’t want to break the bank, I almost inhaled my gum!  Could buy a house or buy DarTZeel!