I've had the same Gk for years now, with the same nagging, "If only this thing was warmer, had better eq, sounded better at lower volume...." issues.
My workaround is the MXR M-80 Bass Di plus. Killer little box, great Di, preamp, runs on phantom power or 9 volt. Warms the tone right up. Even gets a okay dirt tone if you go very, very lightly with the dirt side.
In my search I've tested a number of amps, side by side with the GK. Funny thing is, nothing is that much better that I want to go through the hassle of selling the GK and getting something else. The Gk has run for years without a glitch.
The GK contour and presence stay at zero, really, cutting at 500? That takes you right out of the mix. The thing is a bit bright, but that was the sound 20 years ago for rock bass.
If I had to run out and replace it today, the Carvin BX 1500 or 2000. The Carvin heads really impressed me. Even the BX 500 hung right in there with the GK.
I went with a used Ashley BP 41 pre that has a few eq mods and the Carvin HD 1500 power amp, but kept the GK. I still don't have a deep rack for the new stuff so the GK still goes everywhere. Through the fEarfuls, with the MXR M-80, I can do just about anything.
I'd look at the $100 box and your cabs, before dumping the GK.