Creating an Audio Odyssey via Analog and Digital

It is no secret that I like to knob twiddle; adjusting controls enables me to maintain the illusion that

I can have some sort of influence over transmitted and received signals.  When it comes to received signals, there is always the desperate hope of being able to reduce/notch minute (but nonetheless annoying) bits of noise or carrier remnants.

Before owning my first radio w/some sort of DSP filtering (the classic Yaesu FT-1000mp), a JPS
NIR-12 external DSP unit was brought into my operating configurations for it and the ICOM 7000.
Later, because the FT-1000mp possesses only minimal filtering in the Sub-RX, inserting the NIR-12 into the R-channel audio line provides DNF (Digital Notch Filtering) and passband contouring for BOTH ears. Eventually, a JPS NIR-10 was cascaded into the right channel after the NIR-12.

Next up, a pair of MFJ-752 (Signal Enhancer II) units made the scene.  I inserted the 752-C into
the left channel audio.  For the right channel, a modified 752B was added.  The 752-B modification
was the addition of an old MFJ CW-1 filter board, providing 180 / 80 HZ Cw filtering.  

Even later, an MFJ-784 was picked up, offering even more knobs and buttons to play around with.
The MFJ-784 is unique in that the settings can be saved in a sort of local memory - a crude sort of
e-prom.

The REAL breakthrough came when I discovered the vintage Autek QF-1A analog filters. 
Front-ending it with a classic Radio Shaft 16-band stereo equalizer, shaping the desired passband BEFORE sending the audio to the QF1-A filters.  Cutting off the extraneous frequencies before/after
the desired receive passband reduces the possibility of harmonic overload.

Along the way, a passive mixer unit brings audio from the different radios into one input line,
splitting the audio amongst the various filter units.  The output of the units is then blended into
selectable audio using a classic Rockville 4-channel stereo mixer box.

As you can see, the above filter combinations offer a plethora of knobs to twiddle.

Do YOU make use of external audio filter units?

What Discoveries have YOU made?