Comparing 'Thick As A Brick' and 'A Passion Play', Scott Huntley suggested that whilst APP is the more mature and complex piece (partly just due to genre; fitting the music to the subject matter), TAAB is more unified, and over time the more compelling listen. TAAB is composed of numerous accessible and melodic marches, many reprised in variations throughout. This gives the listener a 'touchstone'; interest is sustained by the variation, but frequent returns to the familiar 'home' prevent one becoming lost, and everything makes musical sense (with the exception of the hiatus at the start of the second half), even if the conventions of classical music are unfamiliar to the listener.
"APP is quite different. Instead of being created as a set of "variations on a theme", it is instead an Oratorio in the impressionist style of the late 19th and early 20th centuries. More like Holst's 'Planets' or Mussorgsky's 'Pictures at an Exhibition' than a Handel or Mozart concerto. The individual pieces do not relate to each other in any direct manner. There is more free association involved - but the music itself is stunning in its complexity and downright ingenuity (especially for rock era material), considering that it is, for the most part, dance music (in the sense of classical dance). It's almost a ballet set to words."
For the most part, the transitions in TAAB are very ingenious, further assisting its coherence. In contrast, APP does not 'flow' so much as simply 'happen'. The instrumentation and the vocals would provide a clue, but a person unfamiliar with the album might hear extracts from 'sides' one and two without registering that they came from the same work. In APP, quite of the few transitions are 'clunky and haphazard', sometimes because they are linking extremely different types of music. One might speculate on an alternative version of the album, with certain sections ending outright rather than being laboriously tied together to make the album a continuous piece. Perhaps the transitions would have been smoother if there had been more time to rework the piece.
Daniel Fawcett questions the coherence of TAAB, suggesting that its repetition of themes with slight alterations was contrived to create a more 'classical' feel:
"TAAB does feel at times like it was a bunch of disjointed songs strung together with 'links', taking random bits, linking them with material that had only a vague connection, and calling it a cohesive whole."
The album included Tull's first use of a synthesizer, showing considerable "skill and beginners luck", in Michael Corbett's opinion. "Many of the tone colors that they got on APP we take for granted in this digital age. But they did it with monophonic analog Mini-Moogs and Echoplex. Very low-tech and more skill-intensive than sequencers."
This is also is the only album on which Ian played soprano and sopranino saxophones, the latter being inherently problematic; Michael Corbett notes that it is hard to maintain exact pitch. "That didn't scare Anderson, who used it to give a bagpipe tone." However, it might explain Ian's reluctance to use the instrument again on future albums, and certainly not live. He played the soprano saxophone on TAAB, APP and WarChild, but then abandoned that too; Michael Dawson suggests that Ian didn't play the instrument enough to develop a strong embouchure, the pressure tiring his lower lip and complicating rapid switches back to the flute.
As Derek Laing notes, the untitled instrumental section linking the two scenes of Act One (from 5:50 to 6:50 on my CD) consists of "some pleasant keyboard leading us to a frantic saxophone solo from IA which lasts a mere 30 secs". Derek considers this to be a flaw of the album, bearing no relation to the previous and subsequent material (which is a resumption of the previous part).
Personally, I've never thought it particularly disjointed, and in terms of the narrative, it might reflect Ronnie's initial panic at finding himself utterly alone in Limbo. Interesting point, though, Derek.
On the album, the Best Friends section has a running time of under two minutes, but in live shows this was doubled by an 'improvised' instrumental section, making this a favourite part of the performance for several of the audience. Lin Sprague, for example, recalls:
What at great, heavy riff! It doesn't come across as powerfully on the
studio album, but live, this was where Martin would open up his guitar and
pour out one of the best solos I ever heard him do.
Carsten Bergmann makes some interesting points about the recurring 'Lifebeats' element (theme?). At the end of The Play, "Steve! Caroline!" is shouted over a slightly out-of-tune cacophonous lifebeat, perhaps symbolising the rather harsh, painful, chaotic, mortal life. This contrasts with the unearthly, light, ethereal heartbeats of the dead Ronnie (Forest Dances 1&2). 'Lifebeats' itself, at the start of The Play, is a combination of the two types, perhaps symbolising the transition from life to the afterlife.
Another heartbeat runs through virtually all of 'Overseer Overture', marking the start of Ronnie's time in Hell, just as the heartbeat of the Forest Dances intrroduced Ronnie to Heaven.
Carsten also makes the interesting observation that when G.Oddie interrupts Ronnie in Act 3, scene 1, he 'speaks' over the same melody as Lucifer does in scene two, but with different instrumentation, a possible reference to their fundamental similarity.
For those who are interested in such details, Martin's guitar for 'A Passion Play' was his Gibson Les Paul Sunburst, later also used for the recording of 'Too Old To Rock'N'Roll...'
Source: the TULL'90 programme, available on their UK tour of small venues.