IMHO, @fastninja12 has provides the most useful insights. Sometimes, the source of dissatisfaction is too high of a crossover point for the subwoofer. Here’s something to try:
1. Determine the manufacturers -3 dB point of your primary speakers
2. Multiply that frequency by 0.7
3. Use that frequency as the crossover frequency of your sub(s)
4. Increase the gain on the sub until it just becomes noticeable
5. Tick the gain down a notch
That means if your primary speakers have a -3 dB point at 62 Hz, try crossing over your sub(s) at 44 Hz. It seems low, but low is good.
If this works, problem solved. If it doesn’t, it was a free attempted solution.