To break it down even further (since I just got another dose of this 'lesson and was again faced with it and thinking about it, 'tone in the ease of mix context is not just frequency' issue by a long shot. IMHO it is more shape
This, really. I struggled to get a bass sound I was happy with for
years. After a lot of experimentation and a bit of gear whoring, I have a very nice bass (a custom I picked up for cheap after someone ordered it, couldn't afford to pay, and the luthier used it as a shop loaner for five or ten years before deciding to sell it just to get it out of the shop) running into
a Sansamp RPM, so the source wasn't the issue, but it seemed like no matter what I did or what I tried, it sounded mushy and indistinct.
What I finally traced it to was a combination of technique and treatment. Basically, I wasn't a very good bass player (I'm a guitarist, and play bass like one) so my basslines weren't terribly even. To fix this, I relied on compression, and ended up rolling off a lot of the high end to clean up the attacks. The result was not pretty.
So, I started doing two things differently. First, I began tracking bass first, so I'd have to really focus on it, and started recording tighter performances. This helped tons. Second, since I noticed the bass sounds seemed "better" when they went through an amp with a bit of grit, I began bi-amping, splitting the signal (after some experiment using real amps) through the Sansamp and recording both the "unaffected" DI output, and a gritty, distorted one from the preamp itself. The RPM's preamp distortion blows, it's kind of overly crunchy and edgy and isn't very dynamically responsive, but layered against a clean DI track (with a little bit of EQ to tame the extreme high end) it actually sounds pretty damned huge. Then, I process the two radically differently - I generally highpass the distorted track somewhere in the low mids and leave it relatively uncompressed, relying on the natural compression from the distortion, and let this track take care of the 'punch' from the attack, and then lowpass the clean one to clear out the high end and leave space for the Sansamp track, and then compress it quite a bit more than the distorted one. The result is a bass tone with some grit, a very clear attack, but a very even and controlled "depth."
If you don't have a preamp like this that lets you split your signal, you can kind of fudge it in your DAW. Either record two tracks from the same DI send, or record one and then copy/paste to a new track. Leave one DI track clean, and then run the other through an amp simulator or a distortion plugin. It doesn't even have to be a terribly "musical" distortion, because you're listening to what it sounds like
with the clean signal, not without. Give it a try!