Back in the 1980s, for instance, Tennant and Lowe made a defining statement in their hit "Opportunities" (1985) - with its chorus, "I've got the brains, you've got the looks, let's make lots of money". On the one hand, the song was seen by some critics as a critique of Thatcherism; on the other, there was a jaunty, singalong triumphalism about the whole number which made you think that perhaps the Pet Shop Boys were enjoying a vast practical joke, in which they became world-famous pop stars by singing about the vacuity of ambition in a culture fixated on celebrity and success.
In many ways, the Pet Shop Boys have maintained their position as the jokers in pop's deck of cards. They always confound the public's expectations of their next move while remaining wholly faithful to their founding image as impassive, determinedly distanced commentators on the human condition. Performing in Moscow in 1993, wearing matching, conical dunce's caps, for instance, they managed to blur the line between performance art and a kind of highly cerebral form of comedy, in a way which no other pop duo could ever hope to achieve. Part Gilbert and George and part Morecambe and Wise, they have managed to develop a cultural identity robust enough for them to take the risks which most other pop stars would neither dare to attempt, nor, most probably, be capable of conceiving.
Initially, their intense minimalism - a singer and a keyboard player, neither of whom seemed to be capable of smiling, let alone moving about on a stage - made it seem unlikely that they would ever be a group who favoured live performances. But, with that neat reversal of logic which distinguishes their creative stance, the Pet Shop Boys have become a great live act through their very determination to dismantle the whole tradition of live pop, and replace it with a form of theatre which can be seen to blend a whole array of contemporary art forms. To this end, they have worked with a succession of artists and artistic directors. Two years ago, they collaborated with the video artist Sam Taylor-Wood for their "Somewhere" shows at the Savoy Theatre in London, performing between a pair of enormous back-projections which enabled them to appear to walk off the stage and join the increasingly tipsy group of revellers on the two giant screens.
With each of these collaborators, the Pet Shop Boys appear to have grown in confidence, allowing themselves, little by little, to luxuriate in the sheer originality of their own particular take on the point and potential of pop performance. Their next move is reported to be a full-scale musical. Now, for their "Night Life" tour, they have achieved the summation of their career to date, in a dazzling display of pop as theatre which sees them at their most audacious, their most confrontational, and, necessarily, their most relaxed.
Working in conjunction with the acclaimed costume designer Ian MacNeil and the visionary architect Zaha Hadid, who has designed a stage set which she describes as "an ever evolving synthetic landscape", Tennant and Lowe have developed a showcase for their greatest hits which combines a post-modern reclamation of gothic fantasy with little less than a futurist interpretation of a music hall knees-up. Imagine a hi-energy disco sequence in Kubrick's 2001 or a production of West Side Story as conceived by Almodovar and you begin to get the picture.
From its opening number - in which they perform live but unseen behind a white curtain on which are projected their rotating, disembodied heads against the fluorescent soundwaves of a radiophonic oscillator - "Night Life" is a triumph of the unexpected. When the white curtain finally dropped, it revealed Hadid's extraordinary sloping stage set, like a late Henry Moore collapsed on its side and bathed in green light. Tennant and Lowe resembled Nosferatu and Edward Scissorhands respectively, dressed in their new image as shock-headed samurai boot-boys, with the heavy black eyebrows Siouxsie Sioux made famous and tiny, beady-eyed dark glasses. The capacity crowd was on its feet in the kind of air-punching frenzy you might associate with a concert by Nine Inch Nails.
But the real brilliance of the production came from the way it released the Pet Shop Boys from the suffocating confines of high camp, allowing the emotion in the music to shine through. In one of the most moving pop performances seen in recent years, Tennant duetted with a back-projected montage of films featuring the late, wonderful Dusty Springfield, for "What Have I Done To Deserve This?", which they originally recorded together in 1987. It was a glorious tribute to Springfield's memory and genius, sung with a love which communicated itself to the entire audience. Tennant concluded the number by blowing a kiss to the dissolving image of a young and beautiful Dusty, taking her bow on a black-and-white television programme.
The mood was then kicked up-tempo with a joyous rendition of the Pet Shop Boys' most recent hit, "New York City Boy", in which the superbly accomplished backing vocalists returned to the stage as tap-dancing US marines, dressed in the snow-white sailor suits once worn by Sinatra, Gene Kelly et al in On the Town. While Lowe made a counterpoint to the on-stage action by remaining so motionless at his keyboard as to become the most conspicuous person on the stage - that old Pet Shop Boy creative reversal of logic again - Tennant really began to enjoy himself, taking obvious delight in his performance as a rock star.
The gothicism of the first half was exchanged for pure Warholian popism in the second. And the defining moment seemed to be the riotous rendition of "Shameless". To images from Hello!, Hola! and OK! magazines flashed up behind him, Tennant conducted an audience in full throat, gleefully singing along: "We're shameless, we will do anything, to get our 15 minutes of fame ..." And then, with the rising roar of a football crowd: "We have no integrity ..."
Some people may question the theatricality of Tennant and Lowe's performance. But it is this very extravagance and audacity - Diamond Dogs for the Microsoft generation - which has enabled them to reinvent the whole genre of live pop.
The Pet Shop Boys' British tour begins at Glasgow Clyde Auditorium, 4 Dec, and ends at Wembley Arena, 20 Dec. Tickets: 0115 9129111