IMO the first problem is thinking the tritone substitution is something exclusive to jazz, and it's just the domain of jazz to say what it is or is not.
Only the name "tritone substitution" is particular to jazz. But the harmonic movement is actually very old and for centuries was called an augmented sixth chord resolution.
Technically there is another difference augmented sixth chords resolve to the dominant chord, but it jazz the tritone substitution moved to a tonic chord. However, the thing that maintains the similarity of the harmonic movement is looking at the half step voice leading.
First the fundamental half step movements of the V to I resolution...
...the fifth is omitted to show only the essential scale tones involved. The red lines show the resolution of ^4 or FA to ^3 or MI and ^7 or TI to ^1 to DO. Both of those movements are resolutions by half step. Even when you remove the roots of both chords the G and the C the remaining tritone resolving to a incomplete tonic chord is recognized as the essential harmony of dominant to tonic resolution, the essential function of the dominant.
Next, the augmented sixth chord resolves like this...
...where the two tones of the augmented sixth interval - the A♭ and F♯ - move in contrary motion, by half steps, to the two Gs of the dominant.
Notice how both of these resolutions involve the dissonant intervals of either a tritone or an augmented sixth resolving by contrary motion in half steps.
Next, the substitution of the tritone substitution is a substitution of chord roots. The root V is replaced by the tone a tritone away, namely the ♭II, or in this example G gets substituted by D♭...
If we break up the tritone substitution resolution into two parts we get...
...now we have both internals resolving by half step, but both resolve to the tonic, in other words they have the same harmonic function, solving to the tonic.
In jazz the augmented sixth (which is common given an enharmonically different spelling) doesn't resolve like the augmented sixth and may resolve to some kind of seventh or extended chord of the tonic, like this...
...but that doesn't change the essential movement of resolving to a tonic chord. Notice that it does effectively make the harmonic movement just a completely parallel motion in all voices by half step. (That will come back up in just a moment.)
We can also return to the augmented sixth chord and see how we simply treat the chord of resolution not as the dominant, but the tonic, it will appear just the same as a tritone substitution, and we can even return the substituted root ♭II to V to get a basic V to I dominant function resolution...
Sorry for the long introduction, but the point is provide a bit of history and make clear the tritone substitution:
- maintains the essential tritone interval resolution to the tonic
- maintains the function of a dominant harmony resolving to the tonic
The substitution of ♭II for V is not arbitrary. It's a substitution that works, because it maintains the dominant function of V to I. The substitution of root by the distance of a tritone (V subbed by ♭II) is not arbitrary either. It has a historic origin in the augmented sixth chord (which is how it first appears in jazz too, found in I Got Rhythm resolving to the dominant), and it provides a half step resolution to the resolving chord root.
So, can you just move any chord in a standard progression by a tritone?
Well, I suppose anyone can do anything they want and then claim any theory label they want for it. But that doesn't necessarily redefine the very well established theory behind what an actual tritone substitution is... or is not.
... The progression before substitution is Gmaj7 - E7 - Am7 - D7. The chord progression with tritone substitutions is given as Gmaj7 - Bbmaj7 - Ebmaj7 - Abmaj7. ...This progression sounds very nice
Harmonic movements can sound nice for various reasons. In this case I found point out:
- you can move these movements by holding one tone and move the others by step with about half of the being half steps, in other words it's very smooth voice leading, and that usually sounds nice
- Assuming the progression repeats you will get
A♭maj7 to Gmaj7, completely parallel motion by half step, which in this context of very chromatic half step voice leading will both sound nice and appropriate.
That doesn't necessarily mean the process used to derive these chords should be called tritone substitution.
There isn't really a specific term to describe it, but the end result isn't necessarily new. Moving chords through a thicket of chromatic half step changes can be found in other styles. Look a Chopin's Prelude in E minor as one example.
Finally, the original progression...
Gmaj7 - E7 - Am7 - D7
...after the tonic chord is just a sequence of roots descending by fifths where the last chord, the dominant could take a tritone substitution...
Gmaj7 - x - x - A♭7`
...the other chords are x just to show the actual tritone substitution is just the the dominant. Those other chord roots can set by recreating the descending fifths progression which some people call "back cycling."
Gmaj7 - B♭7 - E♭m7 - A♭7`
...those two chords could be any combination of major/minor qualities which work as either diatonic relative to the A♭7 or secondary dominants.
I suppose you can call B♭7 - E♭m tritone substitutions or you can view them as the descending fifths progression transposed relative to the tritone substitution A♭7, or "back cycled". The later is how I've usually seen it explained.
