I'm with Shelley on this one, plump for Wrocław.
Pah, it was mentioned in The Independent as one of the 10 up and coming places to go on a pissup. Or something similar...
Poznań is your place. Hardly any British tourists, not many Germans (unlike Wrocław :P) and plenty of places with ??? opening hours. And most of all, it isn't terrifying like £ódz is. The downside is that accomodation is pricey here, though.