I think that must have been the classical/romantic discussion.  Classical works within a form, the play is within (this is my model, anyway); romantic kicks out--into the freedom of...the lack of maybe as you say just the one constraint or maybe more--either form, instrumentation, harmonics.  But yeah, there are natural limits.

I have two friends who are into Free Jazz.  It's not my music--Sun Ra is as close as I get; it took me a long while to understand the sound world (the way I see it: I'm walking through a market of musical sounds, I can concentrate on one bancarella or another, they're not necessarily playing in unison--so it's not a from-many-make-one style of music--it's the overall sound world that counts...that's my take, anyway!

Both are big fans of Eric Dolphy (3:34):

Link: http://www.youtube.com/watch?v=YuiIyDxa750

I don't know what the maximum number of constraints kicked in one go might be, but I always--when thinking of this distinction--come back, not to a musical example, but rather poetry, Kubla Khan, which is, I think (or so far as I have read), unrepeatable in structure--it does all kinds of strange things (it's great to memorise, that way I found all the different rhythmic movements)...I think with "classical" (in my model), at the very minimum you have a standard form to attach to.  The worst it can be is boring.  With romantic, the best it can be is mind-expanding (there are limits, but they are Waaaay out there compared to what the public is used to--think Star Spangled Banner by Jimi), but the worst is....formlessness--a sort of musical sludge...but hopefully the mind-expanders expand enough minds and those minds create forms, structures such that what was once "out there" becomes "in here"--and vice versa, with classical, if the internal innovations are great enough, they break out--the form can't contain the contents so it becomes natural for a next wave of music to break out, following directions indicated by the internal movements...something like that!

Anyways, here's the poem.  Note in particular the way the rhythm completely changes after the ancestral voices have prophesied war.

Kubla Khan

In Xanadu did Kubla Khan
A stately pleasure-dome decree :
Where Alph, the sacred river, ran
Through caverns measureless to man

Down to a sunless sea.

So twice five miles of fertile ground
With walls and towers were girdled round :
And there were gardens bright with sinuous rills,
Where blossomed many an incense-bearing tree ;
And here were forests ancient as the hills,
Enfolding sunny spots of greenery.

But oh ! that deep romantic chasm which slanted
Down the green hill athwart a cedarn cover !
A savage place ! as holy and enchanted
As e'er beneath a waning moon was haunted
By woman wailing for her demon-lover !
And from this chasm, with ceaseless turmoil seething,
As if this earth in fast thick pants were breathing,
A mighty fountain momently was forced :
Amid whose swift half-intermitted burst
Huge fragments vaulted like rebounding hail,
Or chaffy grain beneath the thresher's flail :
And 'mid these dancing rocks at once and ever
It flung up momently the sacred river.
Five miles meandering with a mazy motion
Through wood and dale the sacred river ran,
Then reached the caverns measureless to man,
And sank in tumult to a lifeless ocean :
And 'mid this tumult Kubla heard from far
Ancestral voices prophesying war !

The shadow of the dome of pleasure
Floated midway on the waves ;
Where was heard the mingled measure
From the fountain and the caves.

It was a miracle of rare device,
A sunny pleasure-dome with caves of ice !

A damsel with a dulcimer
In a vision once I saw :
It was an Abyssinian maid,
And on her dulcimer she played,
Singing of Mount Abora.
Could I revive within me
Her symphony and song,
To such a deep delight 'twould win me,

That with music loud and long,
I would build that dome in air,
That sunny dome ! those caves of ice !
And all who heard should see them there,
And all should cry, Beware ! Beware !
His flashing eyes, his floating hair !
Weave a circle round him thrice,
And close your eyes with holy dread,
For he on honey-dew hath fed,
And drunk the milk of Paradise.

Don't fight forces, use them R. Buckminster Fuller.