Need way more info. If you are thinking that a Rumble 15 is the awesome sound you want, why are you not using it on stage?? Myself, I use 850w on stage for my rig. 6 piece band with 2 drummers and 3 guitards. This last weekend I ran direct cause I was filling in for an acoustic duo.. It sucked.. I was playing through a 25,000w PA and had 6 different monitor mixes on stage. Still had trouble with tone (NONE) and getting the feel of my bass through the monitors. I know there are people in here that play this way, but I hated it.. (I used a Radial DI by the way)
Playing direct through a PA cabinet or stage wedge can be a bit unsettling if you haven't done much of it before. Some pre-processing is usually needed. With tone controls wide open and roundwound strings, you'll be in LEVEL 42 heaven, but short of that sound, it is a totally different animal going direct to PA than using a traditional bass amp. Speaking personally, I am not keen on relying only on house monitoring for bass as it can be unpredictable unless you are working with the same crew/gear night in and night out, in order that you can tweak it to your liking and really work the sound. Thus, I carry a full range stage rig to guarantee that I have the chance of getting a tone I like without 100% reliance on house provided gear.
I've been at this for a few years now, and use a front end preamp/processor for any combination of things: amp/footpedal/cabinet emulation, compression, EQ, and other typical special effects. IMO it works well, but hasn't been without a some adjustment and learning curve. That said, the tones being fed to FOH are leagues better than just taking a direct feed off of a bass or out of a bass head, and relying on the FOH mixer to shape the sound. Monitoring with Full Range bass speakers means that the FOH gets essentially what I'm hearing, with only minor tweaks needed if done properly.
All I can say is that this method does work - at least for me - and that the pre-processing strategy is absolutely key to its success if you want it to sound like something that you are used to in a traditional bass amp rig. Pedals like the SansAmp BDDI, the MXR M-80, and the MS60B (which emulates both of these, as well as a bunch of well-known bass amps and cabinets) will get you in the ballpark straightaway.
I will say, though, that a lot depends on the bass. I have a couple basses that have their flaws enhanced going pure direct (no preamp) into a good full range speaker system, while others can sound really excellent just using a little bit of smoothing compression and working the tone knobs of the bass itself. But for certain, this works only with bass tones that are SUPPOSED to be unadulterated (i.e., sound like a natural bass, as opposed to a bass that has grit, overdrive, etc.). If you're looking for "vintage" amp tones, or aggressive metal tones, then some kind of preamp/compression strategy is absolute necessary. Not to exclude simple/generic bass tones either, the harmonic distortion generated by preamp overdrive can really sweeten and fatten an otherwise nice - but fairly sterile sounding - bass tone, especially when run direct to PA cabs.
JMHO.