The first thing you read in most manufacturers manuals or recording books about setting levels is to "get the level as hot as possible without peaking". What they don't explain is how this leaves you with exactly zero headroom on the mix bus when it comes time to mix the track.
Sometimes it reads to get as close to 0 dB without peaking without specifying what type of dB. In a lot of recording books this means 0 dBu which is COMPLETELY DIFFERENT than 0 dBfs (full scale).
The first is from a school of thought that comes mostly from the 16-bit digital recording era where "using all the bits" was important in insuring a low noise floor. With today's 24-bit and higher signal paths this is no longer advisable and in fact detrimental to proper recordings.
If you have already tracked your project this way the best advice I can give you is to use a trim knob or gain plugin on each track to turn the levels down by 10 dB and possibly even further than that. You want to turn everything down equally until your loudest peak with every track playing is no longer clipping the master bus. Unfortunately you may have already gotten some unwanted distortion from your analog to digital convertor and there is no way to remove it except to start over and retrack everything.
There is a better and faster way to set levels while tracking. But it requires some research on your part to find out exactly how your recording hardware was calibrated.
Bust out the manual that came with your audio interface and find the page that lists all of the technical specifications for that device. What we're looking for is a maximum or full scale input value that is listed as + dBu. I will list the values of a couple of different popular recording interfaces here so you can see exactly how varied these measurements are.
Avid / DigiDesign Mbox2: Maximum Input +21 dBu
PreSonus FireStudio: Maximum Input Level (Unity Gain, 1KHz @ 0.5% THD+N) +17 dBu
TC Electronic Konnekt: Full Scale Input Level @ 0 dBFS +13 dBu
The difference in the maximum input level between manufacturers is relatively meaningless and doesn't really mean that one device has more or less headroom than the other. What it does tell you is what the particular designers of the hardware had in mind in reference to the calibration of the analog to digital convertor.
How do I use this value to get a better recording?
What the designers are telling you with this value is that the nominal input value is 0 dBu and their device is designed to work best at this level. The +dBu refers to exactly how much headroom you have before you totally clip and distort the signal. But, it still works best at the 0 dBu point.
0 dBu is a holdout from the analog recording days and most audio interfaces are still designed to interface with older outboard gear from this era. This was an era before instantaneous digital peak metering was the norm and the VU meter was king.
The VU meter has a slightly softer ballistic than a digital peak meter. It was designed to "look good" when you ran a voice through it. It was not very accurate but nonetheless became the standard audio meter on every console, tape machine, and radio.
What we need to get an optimal level is a VU style meter in the box! I recommend Sonalksis FreeG because it's free, it's multi platform and you can make it work in just about every single DAW out there.
Before we track we need to load up an instance of FreeG in our channel strip. Click on the Sonalksis logo and change the ballistics style to VU.
This is your optimal tracking level. This is how you get the cleanest crystal clear audio possible into your computer.
Keep in mind that you still need to watch the digital peak meter to make absolutely certain that you don't hit 0 dBfs but for most sources you'll never end up near that level and when you go to mix chances are you'll still have enough headroom left over to comfortably start your mix without distortion.
You whine that it's not loud enough... TURN THE VOLUME UP ON YOUR SPEAKERS DUMMY! Be sure to read my article hereon how to get the most out of your studio monitors and this will all start to make sense sooner or later.