I, too, have used the PS DS Jr. as a pre-amp for the past week and a half and I would agree with some of the other comments - namely the DS Jr. works as a pre-amp, but only up to a point.
My amplifier must have pretty sensitive inputs (I have no idea since it’s a bootleg Hypex DIY NC500-based unit) because I have no issues using the Jr. as a pre-amp, and at an attenuation setting of 100 the volume is higher than I would really ever need. This is even more so when using the Bridge II as an input for some reason. I seem to be getting about a +3dB delta over the USB input that way, at least with Tidal Hi-Fi/Masters.
As far as the sonic signature and fidelity, I notice no difference between using the Jr. as a pre-amp or plugging it into a few different pre-amps I have sitting around, correcting for whatever it is that each respective pre is adding. For example with a NuForce 7-CH pre, it sounds like an additional volume/gain control was added into the mix, that’s it.
I am not quite sure what someone meant by "accomplished preamp" because a preamp should - according to everything I've ever read - function as a virtual wire with gain. That is, add nothing to the sound and detract nothing from the fidelity. That said, perhaps that poster was referring to the ability to drive a wide variety of power amplifier inputs. In that case, again, I'd agree - there are probably amps out there which would really be better served using a dedicated preamp in between. Perhaps YMMV with tube based setups?
I would guess that if your amp and speakers are sensitive enough it would work perfectly fine as a pre-amp, as that is what I plan to do until something obviously better comes up.