I'm gathering this is just a direct recorded instrument, so I'm going to guess you only have one stereo track? Am I right? If so this will be easy.
After you get done with everything you want to do with it (ambiance, maybe a touch of EQ for some "air" for example), I'd do a mixdown to a stereo track with the levels low like they are. Then load that mixdown into Cubase again -- this is the one where you do your volume adjustment. I'm not familiar with Cubase, but I do use Studio One Pro which was written by a bunch of guys who used to work for Steinberg. You might have a mastering limiter with the program. I'd look for that.
I use Sony Soundforge 10 Pro at this stage though -- it has the iZotope Mastering Bundle (very very similar to Ozone). Here's where I do the final polish on it, like any multi-band compression (not for newbs), harmonic exciter (just to add some "tape sheen"), and use the mastering limiter. You might be able to get away with simply boosting the overall volume so that the highest peak is at -0.3 dB. That sometimes does the trick on a simple track, but if you get clipping trying to do that, you need to use the limiter to get more volume. This will squash the mix depending upon how heavy handed you are. I can get a loud track with just limiting the peaks to -0.3 dB and then setting the threshold to -3 or -4. This is very mild and the track will retain a wide dynamic range. I call this my "craptastic" mastering job.
I wouldn't mess with the level on the Master fader or Main, or whatever it's called in Cubase. Leave that set at 0 on your original track. Never work with the original when you boost volume. Always get the mix set proper, then do a mixdown, and you can work with overall level by loading the mixdown back into your DAW or another mastering program. You want the original available in case you need to revert or change something. Also Ctrl-Z is your friend.
And remember "SAVE AS" <filename_#>. NOT "SAVE". Better to have 30 stages of mixing to go to than one that you want to revert to something that isn't there anymore. Do "SAVE AS" after you've made any major change that you're happy with, because you might not be happy with that change the next day.
Large HDs are your friend.