MB Fusion 500 w/ Aquilar DB112 - want more low end

[SNL60]

Hello, and thanks for taking time to read this.

First of all, I'm mainly a guitarist, but in my little circle of jamming buddies and instrumentalists, I'm the designated bassist because 1) they all play guitar, drums or keyboard and aren't very good on the bass, and 2) I started off on the drums, have always loved the bass, and my rhythmic guitar style translates well to the bass and makes it easier for me to make the transition than it is for my buddies.

I guess the main thing to take away from all that is this: I'm a guitarist who loves the bass and I play bass with some of my guitarist friends who don't like playing it as much as I do. Sometimes I dig the bass way more than the guitar and crave to play it instead of my guitar, but I know a lot more about the guitar and about gear for the guitar than I do about bass, so here I am!

Having said all of that, now that you have a brief background of what I'm all about, here's my issue:

I am going to be playing bass in some live shows soon (small to medium sized venues), so I did a lot of research for a new amp head/cab rig and ended up deciding on a Gallien-Krueger MB Fusion 500 amp head and an Aguilar GS112 cabinet (w/ tweeter). I love the tone and fullness of the sound that I get out of that set up, but there seems to be something missing from the low end, and I personally love to rock that low end when I play the bass, and this set up just doesn't cut it for me.

I want to add a cab to this rig to get some more low end, but I don't know what size speaker or what brand to go for. I am considering getting the Aguilar GS112NT (no tweeter) to couple with my other Aguilar GS112, but I'm not sure if that will solve the problem of boosting the low end, or if it will just give me the ability to make my current sound and tone louder, leaving me with the same issue of the low end not being enough for me. Does anyone have any thoughts or advice about that?

Should I look into adding a 15" cab instead? Or maybe an 18" even? Does anyone here play an 18" cab? I know that Mike Gordon of Phish uses 18" PA's, which is what gave me the idea for an 18" cab, but other than him, I've never heard of anyone else using 18" drivers, nor have I heard one in person or played on one, so I'm very curious about what others think about 18" vs 15".

I'm not a fan of 10's and I love the way the GS112 makes my mid-lows and mid-highs sound, and it is more than adequate for me as far as treble also. I just need more BASS! I wish I had access to all those different cab sizes so I could test it out, but unfortunately, I will not be able to do that.

I don't want to spend money on a cab only to find that it is not good for me, so any suggestions, tips, pointers, advice, etc., that you can offer me would be very much appreciated!

Thanks in advance for all your responses, and thank you for taking the time to read all of this, I know it was sort of long-winded!

NOTE- I originally put DB112 by mistake. I actually have the GS112.