Poor Audio Quality, FlightCom 403 Stereo Intercom

FYI: I am helping a 210 owner install a FlightCom 403 stereo
panel-mounted intercom. After straightening out some
shielding/grounding/wiring/input-level issues, and while testing the
unit, I noticed that the music quality through his headphones sounded
very "tinny", was obviously lacking in bass response, level was too low
to be useable when the engine was running, and it has quite a lot of
distortion. I'm used to good sounding stereo in my own a/c, so I have a
basis for comparison. btw, I homebrewed my own stereo music switcher
from scratch..

I went and got my stereo Lightspeed headsets, no improvement. I went
and got my Sony CD DiscMan CD player to use as a music source; no
improvement. Next we called customer service at FlightCom. They were
very helpful, but I had already tried everything the technician
suggested, with no improvement.

During the call, the technician mentioned that they had recently done a
design change because customers were complaining that the entertainment
level was too low relative to the intercom and/or aircraft radios,
especially when using MP3 and IPOD players. Without really saying so,
he hinted that I might want to undo the ECO change, and offered to
email me a schematic/parts-placement diagram.

I used the supplied data to "reverse-engineer" their intercom. It
became obvious to me that their design change was in-fact responsible
for the distortion. They added two resistors to increase the gain of
the "input" amplifiers, thinking that would make MP3/IPOD players
louder, but what they actually did was to drive the input ampifiers
into saturation at normal listening levels. Undoing the change
(removing two resistors) reduces the gain back to where it used to be,
eliminating the saturation (clipping) and the distortion is gone.

However, that left the entertainment level at the headphone outputs too
low. I added two resistors (parallel with existing resistors) in the
summing network at the main Left & Right output amplifiers, thereby
increasing the level of the entertainment without changing the relative
loudness of either the intercom or aircraft radios.

That still leaves the missing bass response. I analyzed their circuit,
and sure enough, each entertainment channel has three coupling
capacitors in the path that are too small by an order of magnitude
(0.1uF should be 1uF or more, in six places).

Sweeping the audio response from the entertainment input to the stereo
headphone output in an unmodifed unit shows that the low frequency
response is down -3db at 300Hz, and -55db at 20Hz. That is what makes
music sound like it is being pushed through a taught string! I sent an
email to FlightCom to suggest that they need to hire me as a consultant
to fix their design...

In summary, the unit as it comes from FlightCom is unacceptable for
music quality. Even though you could fix it, how many of you are
willing to go through the hassle. In its present state, I can't
recommend it to anybody...