Depends by what you mean by good phones. I have seen people say before 'I have good phones' then they reveal those phones to be Creative Fatality, or similar.
Tbh, for the Z5500 I would say unless you want the D2/X for the extra features, if you are looking solely at SQ, I don't think those speakers would be able to reveal it. I say this because, in a review a group of people compared the D2/X to the D1/DX. The test was blind so they didn't know which card was playing what song. The majority could not tell the difference. On a higher end music system the difference would likely be easier to hear. That is with speakers though, the difference might be more easily heard with decent phones, but I cannot say for sure.