While not attempting to muddy the waters, there are other issues to
consider.The 7600 comes out ahead on some important aspects:
The narrower roofing filter is certainly helpful, and the phase-noise
limited dynamic range at 2 kHz improves at 6 dB / octave.
The return of the audio peak filter (existent in the 781, but MIA in
the Pro III) is a plus.
The ability to adjust the Drive Level on the 7600, and thus the ALC
level from the front panel, is a significant improvement.
(Strangely this existed on the 781, but went away with the 756 series.)
On the negative side:
The IMD on SSB in the 7600 PA is significantly worse than the Pro III.
(Listen on the opposite sideband of a 7600 !)
Impulse noise (clicks, tick & pops) triggers the AGC quite badly in the
7600 (& the 7800/7700), while being virtually ignored by the Pro III.
(The 781 is also immune to this AGC problem.) Yes one can turn on the
blanker; not my first choice for a solution!
Neither the 7600 nor the Pro III has as effective a speech processor as
the much older 781. (Also true of the 7800/7700.)
Other issues that don't impact reception:
The LED back light for the LCD display is a vast improvement over the
fluorescent back light in the Pro III
While the spectrum display is certainly more versatile in the 7600, the
ability to see a 1 uV signal at normal spans (+/- 25 kHz) is inferior to
the Pro III.
Subjective comments:
On SSB if the filters are on "sharp", both the Pro III and the 7600 are
far too harsh and fatiguing to my ears. I prefer the 781 for low
fatigue.
DSP filters on "soft" are acceptable from a harshness standpoint, but
still not as good as analog, to MY ears. (subjective!)
QSK is far better on both the Pro III and the 7600 to the 781.
The K3 is very good in a CW contest, but the ergonomics are not in
Icom's class. With headphones on CW, the audio is OK. On SSB I cannot
live with the internal speaker amplifier. The audio poor at best, but
one can use the line output and drive an external amplifier or use
powered computer speakers.
More subjective ! The AGC in any Icom is more pleasant than the K3,
which I can never get to be transparent (non-intrusive). If the AGC
keeps saying "I just adjusted the gain", then it isn't working right.
At least now you can buy a P3 band scope for the K3, something I was not
willing to sacrifice.
73, Rob, NC0B