@s.elliot.perez Hello s......... I have just read through your thread...... and if I understand correctly you want an endless waveform, without repetitive artefacts.
You mention contrabass. If plucked this will be almost impossible (and pointless?) but if bowed then it should be easier.
The difference with an organ is two-fold:
the organ (a tuned pipe) produces a repetitive waveform (with colour that remains constant)
the organ produces a constant volume
A perfectly bowed contrabass note would do (almost) the same, but a plucked note is not the same animal.
So, do you want to do this "on the fly".......
If you want to produce an endless note (without pitch shift) then you need a looped delay, and as @JohhnyMauser just mentioned you need the loop point to be at a moment where the waveform is crossing the zero level....... i.e. .....no sound........ so as to avoid clicks at that point.
And you need the volume of the sample to be constant so that you hear no "rhythm".
And you need the colour of the sample to be constant for the same reason.
Then you have the equivalent of a "Mellotron"...... the very first keyboard synthesiser....... for one "key".......
The best way to create this sample is in your audio editor, where you can tailor it exactly to what you want........ and then play it in Pd repetitively (in a loop).
If you want to do this "on the fly" the result cannot be perfect unless you can play the perfect note. But you can get closer, as you have found, by "mushing up" the sound with reverb etc after the output of the loop.
You should use a "total" compressor.... so that the sound level and power never varies throughout the sample.
You can use [zerox~] to automatically loop at a "zero crossing".
And the input signal needs to be as perfectly constant in colour as is possible.
And the shorter the sample the better your chances....... but........ then you have a less "interesting" and (maybe) more "metallic" sound.