Does transport matter if using external DAC?

Being that a theory should be testable, why don't you simply connect a number of different transports to a specific DAC and see if you can hear a difference. If you hear no difference, then what does it matter what anyone else says? If you do hear a difference, then either your theory is wrong or your test setup is at fault. Why rely upon "experts" or other audiophiles when you can reach your own conclusions?

I trust my ears.
I can hear the difference when I use different transports with my external Dac, no question about it!
Wadia 20 v Parasound 2000 v Conrad Johnson Transports.
Rank them in order, which one do you think sounds the best?

I think with the evolution of hard disk drives being used as transports reinforces just how important a transport is. I think we have(Audiophiles,engineers,designers)have just began to realize how important the tranport is. With simple non-upsample dacs, no filtering and a very good computer hard disk drive, I think you would be shocked at how good digital (redbook) can sound. I personally think we've been barking up the wrong tree, not that DACs don't make a difference, just that transports I believe make more of a difference. So to answer your question yes, transports are very important!

In theory, you're missing the fact that the ones and zeros are encoded at precisely timed intervals. If the transport fails to convey the precise timing of the data to the DAC a phenomenom called jitter occurs. For credible evidence read the jitter specs published during transport tests.

In practice, you're missing the fact that the quality of design thought, the parts used, their layout and vibrational issues all affect any machine's ability to produce the results desired. For credible evidence test drive a corvette.

Or rely on anecdotal evidence: my CD player with digital out sounded like it was creating a gap between the sound of cymbals and the rest of the audio stream. They sounded disjointed and in that sense highlighted. Installing a dedicated transport eliminated the gap. The sounds of cymbals became seamless and integrated into the whole. Was this jitter or some other quality of implementation? Who knows for sure.

Ones and zeros. Once again a little knowledge is a dangerous thing. As with digital cables, transports matter greatly.