Trying to see what exactly your problem is, i.e. what it is you are doing precisely.
Recording from within software, there should not be any reason the volume is low. If you gain your tracks correctly (- 0db average, less that 3dB or so peaks) you should not have any problems. I do advise using full fader setting per channel when recording. Where in a live environment you might want to have some headroom to play with for effect, but on tape that only sound bad.
When recording from the MCX8000, how are you recording? Analogue recording device on master out? Recording software using the return channels of the MCX8000 through USB?
You can record in something like Audacity (free) and choose line 5/6 (MCX8000) as input channels. That seems to give the master output (no influence of master volume setting!). 1/2 and 3/4 inputs most likely give you either channel 1 or channel 2 (didn’t check, but makes sense). There is also a multichannel input which I got no input on with channels 1 and 2 playing (from engine stick). So I am curious about that one.
It may LOOK like the levels in audacity are low (I recorded with my channel meters peaking at about the 3rd orange LED (one more before red), all EQ and filtering at zero and BOTH channel faders fully up. This gave a -6db level in recording, which works fine.
Should you, after recording, want a hotter track, you can always use the amplify option in Audacity which automatically gains the track to have 0 dB max. peaks, keeping you out of clipping and at max volume.
If you use more “professional” software, like Cubase or Protools, you will most likely have some more input gain staging options so you can set input levels directly when recording. But I wouldn’t buy software like that for that reason only :-).
Final tip. Have a look at the Yamaha AG03/AG06 mixer. It is labeled as a webcasting mixer. Giving you all kinds of neat options to hook up your controller, a keyboard, other gear and one or 2 mics with FX. Better than that it functions like a high-end sound card for your laptop/PC and when you hook up your monitors/headphones you have great way of monitoring what your are recording/broadcasting. Of course you can record in your PC in various configurations. It comes with a light version of Cubase which is great for recording and doing some post-production work. Great value for money. I have an AG03 sitting on my desk next to me, MCX8000 hooked up, KRK Rokit 5s hooked up and couldn’t be happier with it.