Another JRiverMC user here. I was a hardcore Foobar2000 guy for a few years, and I did dabble with JRMC once or twice in between. Once I tried JRMC v17, there was no going back.
As to whether JRMC does any DSP in the background or not (I don't mean the DSP functions of JRMC), I have no idea. But it sure makes music sound much better, esp. in the lower registers.
Those who cannot make out a difference between Foobar2000 (with WASAPI) and JRMC (with WASAPI Event Style), please try playing Jennifer Warnes's "Way down deep" (from The Hunter) on their setups. (Please use a CD or a FLAC ripped from a CD for this.) There is a very defined bassline in this song, and the difference between how bass is handled between the two software is very clear to see from how this bassline is reproduced. With Foobar, the bass is all over the place, completely out of control, boomy and reverberating. But with JRMC, the bass is in perfect control, you can actually hear the twang of the string pluck behind the bass, and there is no boom or uncomfortable reverberation. You don't need a high-end setup that resolves extremely well to hear this: I could make out this difference with PSB Image B6s and a NAD326 (fed by Music PC > Rega DAC). With Foobar, the cupboards in my room used to reverberate along with the bass. But with JRMC, there is no such reverberation.
If this difference was caused by some kind of background DSP in JRMC, I'm all for that manipulation.