Being from the Valencia region, my taste in festivals is biased towards those that heavily feature fire and gunpowder. The Santa Tecla would be an irresistible draw for me, but it's likely that the festival will reach its peak after you've departed.

The festival of Cardona near Barcelona would suit your itinerary
Check the website nearer the time for the dates - it might be the 18th Sep to the 22nd Sep this year... maybe (if those dates are right I doubt there will be any sort of climax to the festivities after the 21st)
http://www.cardona.cat/ca/vila/fires...jor-de-cardona
Or just send an email to the 'oficina de turisme' and enquire directly
http://www.cardona.cat/en/contact

Won't be on the scale of the Tarragona festival, but should be a memorable experience none the less.

Alternatively - I'm not clear on the dates for 2009, but the festival of Segorbe in Castellon/Valencia, the region to the South of Catalonia, might be on during your visit. Again, if interested, contact the local tourist office to find out: turismo @ segorbe . es
http://www.segorbe.org/?go=9caf66fcc...6571dd251a9e2f