Distinctly Digital Forever?

So, just to further my understanding: assuming price no object, does there exist any DAC, or DAC/Pre combo, capable of making my streamer (or the best streamer in existence, for that matter) sound indistinguishable from an FM radio signal,

That FM radio signal comes from a digital source at a low bit rate and is compressed/limited to boot. So, if you’re only getting FM quality from your digital gear, something’s wrong. It should be very easy to better an FM radio signal. There are people who prefer low bit rate music into ear buds over CD quality or hi-res digital on a good system, though.

Something always made me believe that a decent tube preamp could "mellow" or "naturalize" the sound of digital sources.

Dgital sources can sound natural and even mellow, if that’s what you want. I prefer just natural and there are many choices for that.

If your digital is sounding harsh or bright, you’ve got the wrong player.  What CD player or DAC are you using?

@tomcy6 For purposes of this question I am defining "better" as "making acoustic music performed in real space and recorded via microphones to tape or direct disc sound more like acoustic music performed and heard in real space." The FM radio signals are from a high-end tuner delivering programs playing vinyl recordings, so if the source is compressed/limited low-bitrate digital, it doesn't sound that way. Granting the many audible virtues of digital reproduction, I just don't find realistic representation of acoustic music recorded via mic in actual space to be among them.

I think Audio Note DACs your description of what is desirable more than do most other high end digital gear.  The prices of their gear goes pretty high and that range is almost entirely the result of using higher quality parts for the analogue post-conversion sections of the DAC; the digital section does not change much and is deliberately simple—no oversampling, very simple filters.