Well..., 2 dimensional you say, .... Its tough for me to venture an opinion without being able to hear your receiver with my own ears and determining if we hear the same type of sound.
You stated that you had the receiver checked and it measured fine, so that seems to rule out the possibility that some electronic components (capacitors, transistors) need to be replaced.
The reason I asked about the bass response - I have a Model Eight, and althought the unit is certainly built like a tank, I noticed it was rather bass-heavy in its sound, as if the Loudness control was always On to super-boost the bass.
B/F