I have the 40 and am good with the tone. It is its own flavor, different from a GK, different from a MarkBass, etc...
I got my Rumble 40 for low volume home practice only. It may never leave my house.
Couple things that helped me dial in; don't be afraid to use the bright, contour, vintage in combination. You don't have to pick just one. You may even give having all 3 on at the same time a try. If that doesn't get it for you, you may do what I do and use your rumble with an inexpensive little Zoom Pedal.
WARNING - Somewhat off topic from here forward, but does go to the original topic; getting sound out of a rumble 40 that makes you happier with it...
I have a ZoomB1Four bought it here on TB! I had been using it with my previous amp, and ancient Pevey 10" combo amp from the early 90s. Sadly, it died. I liked it just fine, but one day it just gave up...
I just carried using my Zoom pedal forward to the Rumble. It's a really nice utility pedal, especially for the price. If you have access to one to try or even have access to the previous version, the B1ON, you can change the nature of the amp using the models they have in that pedal. The B1Four is a little more tweakable because you can separate the cabs from the amps if it turns out all you need is a cab sim model to get you where you need to be. Also on the B1Four has models HPF and LPF pedals. Both Zoom pedals have models of compressor pedals, a couple different EQ pedals, and just outright effects pedals. It seems counter intuitive to use an amp sim with an amp, but it works for me. - Zoom makes other bass pedals with all kinds of amp sim/pedal models, those are just the two I have owed and am most familiar with and are proven to work for what I want them to do...
I've had good luck just setting the rumble 40 flat, and then tweaking one of the zoom amp models to my liking and then changing the EQ on the amp for final tweaking, which may or may not be necessary.
Hope this helps!!