What makes a bigger change, dac or preamp?

While I, in theory, agree that the preamp is the "brains" of the system, your DAC is at the front of the system. If you have a fantastic preamp and a mediocre digital converter, you will not hear what the preamp can do, only what the dac's abilities pass on to the preamp. And then how would you know the preamp's total capabilities (i.e., its highest level of revelations).

It's like driving a Maserati on Sears and Roebuck tires. You will NOT get the total effect of the Maserati, even though it is more "important" than the tires.
A front end is the "money piece" of a system, and THEN the preamp.
I'm surprised to read otherwise, although, to be fair, Dave Wilson did an experiement with a pair of 1,000 speakers with a very expensive front end, and then reversed it: a pair of Sashas with an iPod. Naturally (and I say this as someone who knows Dave), the second setup won. I would imagine what Dave recorded to use on the iPod was atypical (as in, not typical) of the recordings others would have, so that is my caveat.
If you have, for instance, A Bryston BDA-1 and a Modulus 3A, it'll sound fantastic. If you have a JVC XL Z-1010TN CD player and a Modulus 3A, not so much. (I've had both, so I know of what I speak). EVERYTHING COUNTS, so you're eventually going to want components of equal rank in order to achieve the best sound.
Common wisdom is, though, that the preamp is the "heart" of the system.
You'll have to audition this out for yourself. There are places with 30-day return policies, such as Music Direct. I've bought and returned items, because they just didn't perform as expected in my system.

Yes, the DAC is at the front (and I do believe you have to look from front to end,
since you can't undo what is messed up early in the chain), but what if the
differences between DACs are so small, that qualitative variance is relatively
minor between competent DACs, while differences between preamps are more
significant and easily discerned by comparison when listening? I think this is in
fact the case. But perhaps others might hear things differently.

Pubul:
I'd certainly think that, if the DACs are similar in quality and the rest of the system is "colored" enough, it would hide the differences. That's the problem, isn't it? If a system is built to sound "beautiful," the preamp will most likely reveal more differences. If it is put together to sound "uncolored," then I would say the DACs WILL sound different enough that it is the DAC and then, equally, the preamp, that will reveal differences in the system.
It truly does boil down to that, and your suggestion is every bit as valid as mine.

Beauty or truth - to paradigms for system building, and possibly to key to the variety of opinions we get on the same piece of equipment.

I liked the quote in the February issue of TAS, Neil Gader interviews John McDonald of Audience, and he said something that somehow seems related to this idea:

"At Audience,the Golden Rule is 'first, do no harm,' defining harm as any deviation to the original recording. So high-end to me and to the Audience team is about staying true to the music. Sound-sculpting should be left to the recording artists and engineers"

Do no harm - a good way to describe it, and as we know, preamps can do a lot of "harm", more so than most DACs IME. Oh, then we have cables:)