Help me figure out where this problem is happening

Hi guys,
I have a beautiful marantz 2270 that has a preamp problem. Somewhere before the amp section, the left channel loses a lot of bass and low mids, I mean a ton. I am hoping you guys can help me figure out where the problem might be occurring through deduction with this information:

All inputs act the same, be it aux or tape.
Left channel preamp out is always low on bass.
All of the buttons or balances do not fix this.
The tone controls, including bass, work for the left channel! They appear to function as they should, though the bass level is just a lot lower.

Where do you guys think this might occurring? Any suggestions or thoughts or experience would be greatly appreciated. Thank you!
old machines

ore used ones may have various of problems .....
you have to check :

soldering in tone control area
dirty or broken potensiometers
soldering on loudnes switch
loudness switch
electrolytics around secondary psu ( the one that creates voltage for tone control )
damaged capacitors in tone control circuit ( very rear )
general inspection may be other soldring in other areas or a previous bad repair ( wrong input capacitor or amplifier filter capacitor on amplifier board )

best way to locate if the problem is in the amplifier or the preamplifier is if possible to swap the inputs in the amplifier boards if problem mooves from ch a to ch b then preamplifier is the problem

from all that the greater chance is soldering or dirty potensiometer in the tone control area

( some of them uses a strange pot with one layer and six pins in line ..... this tend to fail easier .... )

sakis ..... www. soon ......