What makes a bigger change, dac or preamp?

I have come to the conclusion over the years that the preamp is the heart and soul of the system and a world class one in a world class system makes the biggest improvements I've personally heard. I would also add that I've never heard a preamp that knocks off my socks that is cheap. I'm recalling that you wanted to spend like 500 bucks. So in this case I would say definitely get a DAC with volume control. I personally own the Quad speakers you have and they sound really good with a Benchmark DAC that has volume control. I know you don't want to buy and then sell again but the Benchmark is readily available used and you can sell it back if you don't like it. If you are set on keeping the Quads and have one source then I would see no sense in getting a preamp. Otherwise if you were thinking of a bigger budget then you would be getting into a situation of possibly upgrading the speakers and getting a good integrated amp that would be a lot better than getting a great preamp for the Quads. I own the Quads and the Benchmark and I've never heard anything better for 1000 used price. Next step up is a better set of speakers and an integrated I think. I'm no arguing that the preamp is not the most important thing in a general sense but with your setup I'd say get a DAC with volume. There are plenty of choices around and under 1000 pick your flavor. I personally like the sound of the Benchmark with the Quads, that is what I'd get and it's an easy sell if you don't like it.

Cables. ;)

I think that power amp is the heart of a system and turntable and speakers are the soul of it.
That said, preamp is very important. Cables are important too.

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.