Those Bose bass module & stick setups - also made by other manufacturers - are great for a solo gig or maybe a duo, but are not effective for a full band setup.
It's an interesting trade-off, which, coincidentally, is something I've been dealing with lately as well. If you are playing mono (as many of us do), and the audience is hearing you only from your own gear (as is also often the case), the single powered PA cabinet can go plenty loud (at least if you're using a mixer ;-) ) but it will have a relatively narrow dispersion (often 90 degrees), meaning that people farther off-axis--which notably also often means other band members on stage with you--may not be able to hear you. My Bose L1 Pro 8 has very wide dispersion (180 degrees), but they don't project as far as my EV ZXa1... so the Bose would provide much wider coverage close to the stage, but the sound would fall off more quickly as you go further back from the stage. (The Pro 16 and Pro 32 go louder, but then you're looking at pricier and heavier stuff.)
If you want both wide nearfield coverage and deep projection of course you could use a pair of typical powered PA cabs, but much of the reason many of us play mono in the first place is that we don't want to deal with the extra gear shlep and the space it takes up and the setup/breakdown time it requires (for additional speaker, tripod, signal cable, power cable). So... I don't know. If the goal here is a single mono speaker, I wouldn't necessarily rule out the Bose approach. I think whether, for example, a single Bose L1 Pro8 is better or worse than a single QSC or EV really depends on the circumstance.
(I will say that the Bose is louder than the EV if I go direct into the EV's line input, but the EV will go louder once I add a mixer.)
I'm saying he doesn't need a personal mixer in addition to the mixer they use for the PA. He just needs to send his keyboards to the PA mixer.
Ah. You were thinking that his keyboard is going into band's PA as well, I was thinking it was not. When he said he couldn't get enough volume out of the QSC, my thought was that he could not get enough volume for the room (as opposed to not being able to get enough volume even to use it as a personal monitor).
You feed a powered PA with a mixer. Any decent mixer has gain knobs for each channel, so just use the gain to bring up your keyboards to an appropriate level. It doesn't matter if the keyboards are outputting somewhere between -10 and +4, the gain knob is there to compensate for this.
That is really the crux of the OP, he was feeding his board directly into the QSC, it wasn't loud enough, and he was asking about adding a mixer or preamp to boost his signal. That's why I was surprised when you seemed to be telling him not to do that, since as you've said here, usually powered PA speakers are indeed fed by a mixer.
I think you guys are overcomplicating this. What I do in my band is simple and it works great:
I put my keyboards into a Boss EQ pedal which I use as a volume boost. From there it goes into a Radial ProDI. From there it goes to a channel on a Mackie ProFX16v3.
...which then goes out to the mains, with your keyboards coming back to you via a monitor feed, right? That kind of approach is ideal, but can be tricky if you don't have a sound person. At any rate, "simple" is relative... that's not as simple as what the OP is doing... feeding his keyboard into an amp, done. ;-) But he probably does want to add a small mixer.
And btw, regarding classical music, improvisation was a big part of it prior to the early/mid 1800s. It was only in the mid/late 1800s that playing "exactly as the sheet music" became a requirement.
I can see that for solo work (or solo passages), but when you have an ensemble piece? I would be surprised if they told the string players (or their piano accompanist), "okay, here's the general structure, and within that, you can play whatever you want." I could see that for modern avant garde, though.