If you would like to keep the pre, the mid bass emphasis can be mostly corrected by replacing the output coupling caps with larger and better quality electrolytics. There are also 2 inter-stage coupling caps near the motorised volume control, but these are more difficult to get at. If you can get someone to replace all 4 of these with HQ electros then the bass will be perfectly tight and rock solid, but replacement of just the output caps will give you an 80% fix.
See also my reply to the question of distortion.
Hope this info helped.
See also my reply to the question of distortion.
Hope this info helped.