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"THE longest bull run in a century of art-market history ended on a dramatic note with a sale of 56 works by Damien Hirst, “Beautiful Inside My Head Forever”, at Sotheby’s in London on September 15th 2008. All but two pieces sold, fetching more than £, a record for a sale by a single artist. It was a last hurrah. As the auctioneer called out bids, in one of the oldest banks on Wall Street, Lehman Brothers, filed for bankruptcy.

The world art market had already been losing momentum for a while after rising vertiginously since 2003. At its peak in 2007 it was worth some $65 billion, reckons Clare McAndrew, founder of Arts Economics, a research firm—double the figure five years earlier. Since then it may have come down to $50 billion. But the market generates interest far beyond its size because it brings together great wealth, enormous egos, greed, passion and controversy in a way matched by few other industries."

I’ve been confused by the difference between ‘simple past’ and ‘the past perfect’. Will there be any change to the meaning intended by the author if we put the sentence in red in such ways as ‘The world art market lost momentum for a while after rising vertiginously since or ‘The world art market had already lost momentum for a while after rising vertiginously since 2003.’ Thank you for your help.
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‘The world art market already lost momentum for a while after rising vertiginously since 2003'
‘The world art market had already lost momentum for a while after rising vertiginously since 2003.'

No difference in intent. The past perfect is used for emphasis of precedence, but I do not see its need in the excerpt you have quoted.