Pretty sure shampoo's aren't going to make much of a difference overall, ive used Nioxin myself and find it a good shampoo and i love the smell, but as for it's DHT lowering properties i wouldn't have a clue as i take meds, but in all honesty i doubt it makes much of a difference to DHT levels.
Iv'e heard some promising stuff regarding RegenePure shampoo and like the brand and use it myself but without actually coming off meds to see if it does help i wouldn't really know.