The DS has a fixed noise floor no matter what volume it’s set at - if you have too much gain in your system as a whole then you’ll hear the DS’s noise floor in your speakers and it won’t change as you change the volume. The DS has a better S/N ratio than CD’s can represent so there is a system gain that lets you listen as loudly as you want an still turn the volume down as much as you want. We put the 20dB attenuator in to help with systems that have audible hiss at the speakers to find the sweet spot in gain. There are some systems in between where using balanced analog connection may help by get 6 more dB. And some people have gone the other way and bought, say, a 10dB inline attenuator.
With a preamp none of that matters - you can dial the preamp in and then use the DS’s volume or the preamp’s volume, which ever your mood wants. (If you don’t stress about it your subconscious will figure out if and when to use which volume control.)
There’s no magic way to read a spec and calculate number that will tell you whether a preamp will sound better in your system or not. FWIW PS Audio has a no questions asked 30 day return policy if you find that you don’t need or want a component - if you are interested in whether a preamp sounds better in your system and can’t borrow one (or whatever brand) from a local distributor or friend then you might consider taking PS Audio up on the 30 day trial.
In a system that you upgrade frequently, you will almost always need a preamp some times and possibly not at other times. I think of them as universal adapters between your sources and your amps - there just isn’t a set of specs that all manufacturers builds to so that any component will work with any other. A preamp can iron out the differences.