Are Preamplifier’s Relevant Today or just a Hinderance with Digital Playback ?

Agree with jond and Charles...gotta have an active preamp!

Bought a Project Pre Box S2 as a digital headphone amp but now also use it as a pre from computer USB to amp and it's surprisingly good- even automatically alters volume when phones are connected/ disconnected

There are some very high quality integrated amplifiers, both old and new, and most would be quite happy with the results.  Often outclassing some separates.  However, the best performing systems still use a separate preamp and power amplifier.  

I suppose it could depend on how many sources. I have no use for a preamp. 

You just need to match what is called  consumer line level which is nominal 310mv RMS to your power amp sensitivity. Whilst CD players boast 2v figures this does not occur, as all the media you can find will be 310mv RMS, so your power amp needs to be close to this too.
https://en.wikipedia.org/wiki/Line_level

The long and the short you can then use passive attenuation which is represented by resistance, vs adding via active preamps which can't help themselves but add various flavors of reactance, in so doing distancing you from what is possible.

The benefit is you will hear what your source component can actually do,