I want more bass…

Bass Counts!! 

Are those speakers bi-ampable?

If so buy a Class D ganfet and power them.

Better strategy is to buy 4 good subs and power them with another amp.

You might want to try and run a 50hz tone into the system. Lots of options on YouTube for frequency sources.

Then walk around the room a listen to what happens with bass nodes, peaks and cancellations.

Its surprising that you are lacking bass, so as has been stated above, could be the room and speaker placement.

Just a thought before you start shopping for stuff. I’m by no means arguing against implementing subs, this experiment should help provide some information about how the speakers are interacting with the room.

What kind of music are U listening to? does it have really deep bass? Most people think mid bass is deep bass. Woofer position, as mentioned , is critical for deep bass. Rooms actually have null spots where there is no bass if your woofers are there. That's the plus of subs. You can put them in the good spots for bass and the main speakers for best sound and it's usually not the same spots. Finally how loud do U listen. It's very hard to hear deep bass at lower levels. The ear frequency response varies with level and is only flat at around 100 dB, too loud for safe listening. At normal loud levels bass is down at the ears significantly. 

Experiment with speaker positioning.  If you you don't have much freedom to move them about relative to corners and walls (closer or farther) then you may be stuck with the bass you have.

A sound level phone app and a source for playing full bandwidth pink noise will validate the best spot for the speakers.  

Once you have the positioning optimized and are still not happy with the bass then it is time to consider other options but you aren't there yet.  

Definitely a fan of powered separate subs.

just moving the subs around had a big impact.

Active crossover, then bi-amp, move sub freqs off of main speakers and amps don’t need to try to push those frequencies either.  For my old ears, I also limit the top end frequencies.  I can’t hear them anyway so why should I have my amps trying to push them? (Personally, I limit high end to 18khz, I can’t hear anything up there anyway.)

i know, I know.. but “I” cannot hear the difference, so I prefer to enhance what I can hear.  
I like it this way.  Seems my amps and speakers do also.