For an all-digital system, the miniDSP SHD Studio should suit your needs. It is digital in, digital out, and can be dropped into a system while leaving the rest of it intact.
For a system with analog sources, it’s more complicated, because there have to be ADC and DAC stages somewhere, so dropping a box into "any system" may not be the optimal way of doing this. It reduces the number of conversions to use DSP that is built into a DAC-preamp, like the DSPeaker X4, Anthem STR, or miniDSP SHD (not the Studio version), to name just a few.
You mentioned sample rate. I recently got the Anthem STR Preamp as my third or fourth preamp with DSP. The DSP sounds cleaner than previous models to me. I don't know how much of that is due to using 192 kHz as the DSP frequency, and how much is due to other improvements over past models. A long-distance friend with the X4, which I believe does DSP at 96 kHz, says that it, also, sounds better than older units to him.