My advise: forget it! They were developed for DPL (Dolby Pro-Logic) and are limited in their upper frequency range, I think they cut off around 8kHz already and thus are not suitable for "digital" surround sound which covers the entire frequency range from 20Hz - 20kHz.
That said they might not be bad speakers but rather not suitable. The price of a new pair was around GBP400 if I remember correctly and I always found this way to expensive, THX logo or not. If you still consider don't pay more than GBP50 for a pair ...