I've received my X8, the hum is a definite issue. I get spikes at about 25,384 Hz, 50766 Hz, and 76,143 Hz (plus a wee something at just under 90 kHz). These first three spikes are clearly some sort of harmonic series. The issue is greater in the right channel than the left (and in track 4 than track 3, I have not yet tested tracks 5 & 6). It doesn't seem to matter if the external mics are close to the recorder or several feet away, as far as I can tell.
Tascam mics on the body of the recorder: My noisy room background -117.4 dB; 25384 Hz (L -110 dB) (R -96.1 dB); 50766 Hz (L -111 dB) (R-99.6 dB)
with screen set to dim and backlight off 25384 Hz (L -111.2 dB) (R -96.3 dB), no real difference as far as I can tell.
Nevaton MC 59/W in inputs 3&4 background at -114 dB; 25384 Hz (L -113 dB) (R-100.1 dB); 50766 Hz (L -113.3 dB) (R -100.3 dB) dim or bright screen, mics near the X8 or a few feet away, no difference I can see.
With no mics in 5&6 I still see biggish but not so sharp peaks at those frequencies.
I usually record at 96 kHz, so the 50k and 76k spikes won't mean anything to me, but the 25384 Hz spike will be a concern. Those who record at 48 kHz won't have any issues here at all. I'm waiting to hear how Tascam, who has been made aware of the problem, will deal with this. I have not yet upgraded to the new software, as it does not claim to do a fix for this. Not sure how software could!
Jeff