I mean, mids are basically free. What exactly are you trying to achieve that you're not reaching with full range knob adjustments on the amp? Is there an issue with just the straight bass into the cab, or only with pedals? If there's a particular range you're looking for and not getting, once you've exhausted the possibilities of the amp, you could try an outboard parametric EQ pedal (or graphic, but if you have something really specific in mind, a parametric might be better). There are loads of different ones out there. I have a Carl Martin 3 band that I really like, although I use it for guitar, not bass. 3 semi parametric bands and an overall level control. The bands cover the full musical range basically, so it should work for bass too. That's probably the easiest way if you like the overall sound of your bass, amp, and cab together and just need more of a certain range.
But I just looked up your amp to see the controls. You have 4 semi-parametric bands of EQ, PLUS high and low shelving PLUS ultra low and hi AND bright switches. So I think you need to spend some more time tweaking. Like Chris said, set everything flat on the amp, no effects, no switches engaged, and no compression. But I would go through every band of parametric eq invidividually. One or more of them should at least indicate to you what you're most looking for. Go through each band with the gain maybe halfway up (ie close to 3 o clock) and sweep the frequency up and down through the range of the knob. Make note of any frequencies that makes particularly good or bad. Then reset gain to noon, and do the same with the next band until you've gone through all 4. Do all this with the tone on your bass on full, and the attenuator on the cab at "0db". And don't be afraid to crank up the tweeter all the way up on the cab and compensate as necessary with EQ/tone controls on the amp, as they likely have different slopes and frequency centers.
If you still can't get there after that, then I suggest trying whichever EQ pedal takes your fancy. If THAT doesn't work, then yes, you probably have to change to a different cab. The GK 12 based cabs are cool, but a completely different sound. LOTS of mids, but not a ton of lows. But the mids are pretty fat and nice and the highs have a nice character to them as well.
But I just looked up your amp to see the controls. You have 4 semi-parametric bands of EQ, PLUS high and low shelving PLUS ultra low and hi AND bright switches. So I think you need to spend some more time tweaking. Like Chris said, set everything flat on the amp, no effects, no switches engaged, and no compression. But I would go through every band of parametric eq invidividually. One or more of them should at least indicate to you what you're most looking for. Go through each band with the gain maybe halfway up (ie close to 3 o clock) and sweep the frequency up and down through the range of the knob. Make note of any frequencies that makes particularly good or bad. Then reset gain to noon, and do the same with the next band until you've gone through all 4. Do all this with the tone on your bass on full, and the attenuator on the cab at "0db". And don't be afraid to crank up the tweeter all the way up on the cab and compensate as necessary with EQ/tone controls on the amp, as they likely have different slopes and frequency centers.
If you still can't get there after that, then I suggest trying whichever EQ pedal takes your fancy. If THAT doesn't work, then yes, you probably have to change to a different cab. The GK 12 based cabs are cool, but a completely different sound. LOTS of mids, but not a ton of lows. But the mids are pretty fat and nice and the highs have a nice character to them as well.
). The only thing I miss are some nice mids.
D). 
