If the ahem DVD is a direct replicate of the ori, then it should be fine. Most ori DVDs are double-layer (DVD9), so if the ahem is single layer, most probably they've re-encode to lower bitrate, resulting in inferior quality.

How to know? Just insert the disc into your DVD-ROM drive in your PC, and see the properties. If it's more than 4.3GB, then it's double-layer.

And most DTS track on ahem DVDs are rubbish, they've used the original Dolby Digital track to re-encode to DTS, if they set the settings right, it could sound OK. If the source original DVD already have a DTS track, then that could be the real thing.