yes, here in Bavaro Banco de Progreso is the representative office for Amex, i don't know if that's the case with all of their offices on the Isle, but would guess yes.
the few times i brought clients there to take money from their Amex or cash travelers cheques they always only got paid in Pesos, no US cash, and the exchange rates been not very good, in other words "they made some nice %%%" out of the changes.
i would never exchange money at a official bank, their exchange rates are compared to the Casas de Cambios very poor, on the other side for exchanging small amounts of a few K's it is not worth the hassle to travel around and risk to get scammed if not familiar with the procedures.
here on the east the casa de cambios usually accept TC's only from known customers. some work online to check the TC's authenticy right away before they hand over the money, those bancas exchange the checks for everybody, passport or cedula required of course.
one of the bancas working online to check the TC's is i.e. Banca Barrican in El Cortecito Bavaro.
in general i agree to the advices above,
bring US$ cash or use a debit card.
using a debit card at a ATM provides always dominican pesos, but for the best exchange rate you can get anywhere.
always only use the ATM's attached to a bank office, not some stand alone ones somewhere out on the streets, we had scams there.
if you are visiting for vacations and want to use TC's to pay for excursions etc, that's no problem, tourproviders/outfitters accept the TC's the same than cash, no extra fees or such involved.
cheers
Mike