This is a pretty subjective thing since I can't be sure of the tolerances of the equipment, but the general rule is that you always want the source signal to be as loud and clean as possible. Typically with consumer electronics, such as home and portable CD players, this means setting the device's volume to somewhere between half-way and two-thirds of the way up. Next you want to set the trim knob of each of your mixer channels so that the meter lights stay consistently lit up to 0 dB and only flicker above 0dB. The most important thing to remember when setting the channel trim knob is that the channel's clip light should never ever light up; if the clip light comes on during loud moments turn the trim down until it doesn't light during loud moments anymore. Now it is simply a matter of getting a volume balance between the CD Player and the Tonelab using the channel level knobs and then setting the headphone volume to a comfortable listening level.
Recap ...
- Loud & Clean Input Signal
- Trim knob set so the clip light never comes on
- Meter lights reading very slightly more than 0 dB
- Good volume balance using the channel level knobs
- Comfortable listening level using the headphone knob
It's hard to get more specific or precise than that with typical home recording and consumer electronics equipment. But as long as you let your ears be your ultimate guide the steps I listed will at least get you headed in the right direction.
My Gear ...
Roland Micro Cube, Traynor YCV20WR
Ovation Cedar Balladeer, "Super-cute" Fender Strat, Squier M-80