Funny you should mention the Rumble. While I was in the store picking up the Voice live Play ( I took it back see * )   I was also inquiring about sub woofers. I just sold my Yorkville 200Wx 18" , way to big for my tired old bones. Small subs are like $1,000??. Being a lot smarter than I look, I asked about small bass amps, I got to try a real nice Fender Jazz fretless into the Rumble 12" Wow!!. It weighs like nothing too! What have they got hidden in that thing?  I will pick one up later once I've paid off this last round of GASing up. 
 
And don't knock the idea of using a bass amp for a small sub.. think about it. What is a small sub? A power amp and a heavy duty speaker. What is a bass amp.. that's right. All you do is turn the MID and Treble down voila! a pour mans hi pass filter. I speak from experience. 
 
My plan for the future is to use a DAW , Sonar if so, for my backing track playback. This will allow a few cool things. Audio and then MIDI control of my Effects, and sending the bass track, that's right!  a real bass, to the bass amp. I've done this successfully already with split stereo wave files. It's cool, it is like having a real bass player on stage..except this one does not drool. 
That is why this thread got me going in the first place. Looking at tha XR 12 brings this a little closer to reality. I could use any old interface, but as Jim just said, most interfaces have crummy pre amps. Pre amps and good effects are important to me live. I just bought a Beta 58 too. 
 
* http://forum.cakewalk.com/Lexicon-MX-200-as-a-VST-plug-in-m3111262.aspx
 

The X32 is the best bang for the buck in digital mixers.  I would recommend it above the Studio Live mixers which are in the  same price range.  I'm obviously not going recommend it or compare it to an Allen Heath digital board, or a Yamaha, but those are 3 times or more the cost.
 
I realize Behringer had issues in the past, but the stuff they are coming up with now is game changing stuff.
 
The people scrambling right now are PreSonus, as well they should be who have developed products that look great on paper, but are problematic, difficult to use, and offer little to no support.  Behringer is constantly updating firmware, and bug fixes.  Presonus outsources all their software engineering, so once they've released a couple of patches they never offer software updates.

I've had an XR18 digital mixer running with Sonar for several months now as an 18x18 USB interface on ASIO driver.  For the most part I absolutely love it.  There are some instances where I see a little difference in behavior from Sonar while it's been in the place of my RME, but those seem to be on projects with a mix of 64 and 32 bit plugs, so it's hard to attribute 100% to the interface.
 
I did go from PCIe to USB 2, which really may not even be a real factor for my lightweight use, but it did give me pause.  The GUI interface, which just got an update today BTW, is gorgeous and intuitive, as it's just like a live mixer.  I've got four independent stereo effects busses and compression on each channel as desired for monitoring.  I can run headphones and three stereo monitors off it, and it interfaces MIDI to my Kurzweil.  It also does some kind of ethernet P16 monitoring, which I've not even looked at yet, but I understand adds lots of monitoring flexibility with some add on devices.  Then I can gig with it, which really is it's primary function, and for that it's just incredible.
 
At home and gigging I have it hardwired to an ethernet switch with simultaneous control from whatever it sees on the network including IPAD control VIA WIFI provided by the LAN.  I've not used the internal WAP on it as I prefer keeping a wire on it.  I see the IPAD control as icing on the cake and not required since you can control from the computer.  But it has worked great and I'm happy I got it.  Almost sent it back because I initially thought it was controlled *only* from the IPAD.  Then realized I could run it over ethernet from the computer, then I saw the interface and realized the power it had, so I kept it.  The preamps sound meaty, it's responsive, the processing sounds great, today's update looks fantastic, hence I've decided shared my experience.  They've not put out an updated ASIO driver since the first one, so I hope that's approaching soon.

THis is good news,, now you got me going again on this!!

Cactus Music
THis is good news,, now you got me going again on this!!

There's a fantastic X-Air group over on Facebook that has tons of really good information and video tutorials that demonstrate all the box is capable of, so you can really get a thorough tour of it.  Much like the vibe here, its a very friendly and well informed group with users from everywhere.  My only quibble was going to USB2 from PCIe, which is likely negligible for my use, and now I can use it on my desktop and notebook and anything I may get in the future.  As I mentioned, I almost sent mine back before truly realizing all it could do, plus direct PC control via Ethernet with nice GUI...my nine year old daughter can run it.  And I've never had 16 pre's in my life, except for my Mackie CR 1604 mixer back in the 90's...don't even know yet what to do with that many.  So that, plus tracking with monitored compression, limiting, gating, FX, dadadadada...I'm enjoying it.

I will add here also that the ASIO4ALL driver is doing way better than the Behringer driver.  I know there's better, but even with my seven year old hardware listed below, I'm getting 10 ms round trip, which I'm happy with.  And it's only pushing hard when I put CPU hungry plug in on the master bus.  In this case Ozone 6...makes a big difference on the CPU when on or off.  I'd love to see it on a late model PC.