Gain staging is pretty simple but a surprising number of people don't get it.
Generally there's some kind of way to meter the sound level going into the preamp. Should be a clip indicator or something like that at least. Crank the gain up until the light starts to flicker and then back it down so it stops for the most part. If you see the occasional blip you're probably okay and this should put you somewhere around line level, hopefully.
The manual should say something similar.
With any luck this should put the average or RMS signal level somewhere around where the manufacturer calibrated it to. Different systems might calibrate the converters anywhere from around -15 to -22 dBFS = line level, RMS. These are average levels, not peaks.
Sounds like what you're doing is probably okay if the tone is good. Going conservative doesn't hurt.
If by "soft" you mean "not as loud as a commercial CD", you're doing okay. You can boost the signal level in your DAW after it's been recorded if you want or need it to be louder. Commercial CDs have been compressed and limited to death because of the loudness wars. It doesn't help the sound.
If you print your tracks too hot you can damage the sound. Be careful with distorted guitars - there isn't much in the way of transients on that type of source, so it would be easy to overcook the signal if you're just looking at where it peaks in protools. -6 is way too hot.
