I guess it depends on how high of a sound quality you are after? Having used and owned some very good DACs and also some moderate dacs, such as those indicated above, you will find that a good transport will make a big difference over a cheap and not so well made transport. I know this from both my personal ownership of equipment as well as listening to other systems as well. There was a time that I thought this made sense (that the quality of the transport was somewhat not material) and that the DAC was the only impacter of performance. It took me about 2 seconds of listening with a good transport to realize this is not at all the case. If it were as easy as just buying a cheap transport and a mid quality DAC then the sale of very good transports for much higher prices could not be justified. I, for one, would much rather get comparable sound by spending much less money. I live in the midwest and there hasn't been but one person who has entered my house in the past two years that has ever heard of a single brand in my system - so the idea that I would spend extra to impress somebody does not make sense. Most people that enter my house would be much more impressed by seeing a stack of Sony equipment (for example) than my existing system. So in my case, spending more from an ego position just doesn't make sense or hold water (just to prevent that comment).
So in the end, a better transport will make a difference, whether you select a mid level DAC (such as those mentioned by Bob above) or a super high quality DAC that can cost as much as a car - EVEN WITH A HEADPHONE RIG!
By no means am I suggesting you will not be completely satisfied with the DACs mentioned above, they are both admirable DACs (especially for the money) and may, as Bob has indicated, incorporate good headphone amps (regardless of what the rags reviewers state - which never includes anything but a positive statement). However, the claim that there will be NO difference between a reasonably good transport and a mediocre transport will become immediately apparent upon listening. The benefit of headphone listening is that you can reduce a lot of the costs of a system (cables, other components, room issues) to get truly great sound.