The issue you describe seems an impedance adaptation problem. If you can afford for I recommend the use of an specific appliance.
I have been testing a lot of mountings for doing vinyl & tape recording (I have about 3000).
My target was to record at 96 KHz and 24 bits. What I tested is:
- Behringer FCA202 which is a ADC & DAC converter with firewire connection at a good price (less than 100 $).
- Apogee Duet. Also a firewire ADC & DAC appliance. It is an appliance with a outstanding audiophile sound. Do a WEB search to have details. Price is about 500 $, a bargain considering very high quality sound.
Both works fine with OS X and Audition for Mac.
On audition you can record in 32 bits, apply effects and store in 24 bits at the end with dithering activated. This gives geat results.
Note that these two devices performs ADC and DAC functions.
Hope this helps
This is not an Audition problem. I forget what features OSX 10.1 had, but in modern versions of OSX, you can choose between LINE and MIC in System Prefs. You're obviously using the MIC setting where LINE is really what you need to use for amplifier signal levels.
I do the same things that you do (clean up records and tapes). To echo the previous poster, the best way BY FAR is to digitize with an external box. I use a Roland UA-4FX (about $150). It's USB. It does a great job and is compatible with all of my Macs and PeeCees. Also works with Audition 1.5 and 4.0 beta without complaining. I don't know if it will work under OS 10.1, though.