I have my DIR-655 and an ADSL2+ modem provided by the ISP.
"ZHONE" printed on top. Label at the bottom reads: XAVi Technologies Corporation X5821.
I am not experiencing any issues between the router and the modem.
My only issue has been initially that the 10mb connection with this new modem seems to be more finicky than the 5mb one wit the old clunker of a modem. After the ISP tweaked the modem twice the connection seems very stable now. But again, that was only an ISP networking issue towards the modem.