Interesting, IT03 mids seem quite deficient compared to ’07. Jerry Garcia’s voice sounds thin and nasal by comparison (Uncle John’s Band - Workingman’s Dead, Friend of the Devil - Dead Set). Of course there are those who say his voice
is thin and nasal

But I prefer the warmth the ‘07 adds.
And still there’s that sense of ‘better’ — bass timbre & texture, the ‘liquid’ quality of Jerry’s guitar (again from Friend of the Devil).
I guess I would call the ‘07 sound ‘richer’. Mids come up, but neither highs nor lows suffer as a result. In particular, I would call 07 bass every bit as powerful as 03 bass, perhaps a bit more. London Grammer Hey Now has some sort of synth bass which reaches very low — I literally feel my frontal sinuses vibrating along with that track.
Nick Drake’s Hazey Jane1 from Bryter Later features some sort of ‘rolling’ percussion. Just as prominent from 03 vs 07. But more well-defined from 07. 07 tonality is ‘further’ from 03 than from 04 — primarily in the mids. On Hazey Jane 1, the strings come up a lot on 07 vs 03. And his guitar is fuller and more present.
Headstage is another dramatic difference. I actually think the 03 headstage is more coherent & natural than that of 04. But it has a distant quality, and there’s a sense of ‘music-in-a-barrel’. Conversely the 07 headstage actually approaches the ‘That Instrument, Right There!’ precision of the Utopia headstage.