Satoshi, I think you are right. I have tested the Retrobit a little this morning and I have come to the conclusion that the stock S-Video cable that comes with the system is crap. On NES, the picture is flawless through the A/V on my HDTV, but when I plug the S-Video cable, the screen gets grainy with diagonal jailbars and a small amount of static. Obviously, the S-video cable is a very thin unshielded wire, so I replaced it with a known good premium shielded S-video cable.
Here's the real kicker: with the shielded S-Video cable, the reception gets even worse! More static, NTSC colors barely show up, and worse diagonal jailbarring. Genesis is not as bad, vertical jailbars using the stock S-video cable. Jailbars are even more pronounced using said premium S-video cable. Genesis audio is definitely cleaner than the A/V multi-out or RF output on my stock VA7 Model 1, but this revision in particular is known to have grungy sounding audio. Both NES and Genesis are bright and vibrant with no static or jailbars when using the regular A/V. SNES displays flawlessly through either cable.
But shy the bloody hell is the NES S-Video actually worse viewed through a premium shielded S-video cable than using the stock POS cable? I'll have to get a video of the craptastic effect later.
Also I tested the drum synth on the NES using track 7 (Money) of my MOON8 cart by Brad Smith (8-bit Pink Floyd DSOTM tribute). The sampled cash register sounds a little muted and doesn't ring right compared to the real NES or A/V Famicom. All in all, the slightly off samples aren't nearly as bad a deal breaker as the swapped duty cycles that plagued clone systems for years. Bass is definitely audible when played back on HiFi speakers, comparable to my NES and stronger than the A/V Famicom.
One more thing to add, Stereo separation is flawless on both the SNES and the Genesis. NES is dual mono, as it should be.
EDIT: Sorry for the triple post...