Hi Mark,
The thing about shag piles is that they are hard for Mrs Housewife to maintain with her 'oova, consequently you will probably be faced with a tangled pile with a soil build-up at pile-base.
For best results some pile treatment is often required prior to the pre-vac, be it with manual or mechanical means (eg. Host -type machine), open up the pile prior to the dry vacuum, which should be thorough.
A micro-splitter would be ideal, due to the lightness of pre-spray required and the fact that there is no harmfull residue.If you have a choice of wands, choose one that has a fairly wide slot as this helps to straiten the pile-strands. I like to set these piles with a final drying stroke rather than by brushing later. The finished appearance has every bit as much to do with pile treatment as to clenliness. Turbo drying is even more beneficial with long piles than with shorter.
John.