the pilot 408 is a receiver, no? the multiplexer allows you to receive stereo broadcasts from the mono fm tuna, thru its mpx output.
pilot gear is absolutely excellent, imo - their amps, preamps & tunas inside their receivers are highly rated amongst wintage enthusiasts as amongst the best sounding gear awailable, as are their separates. query a/c member whitese about it. i am listening to a mint fa-680 fm tuna as i am typing this; its sound is absolutely superb, as is the sound of the pilot 580 tuna i also own.

the fa-680 looks yust like the one in this pic of whitese's former pilot gear, but w/o the fancy wood cabinet:
there's also a yahoo forum dewoted to pilot gear, (
http://groups.yahoo.com/group/Pilotone/ ), as well as lotsa info about it over at audio asylum and audiokarma...
hth,
doug s.
ps - i have a pilot 660 tuna w/non-working meter that should also sound excellent, as it's similar in construction to the 580 & 680; i am willing to give it to anyone who is willing to pay for its shipping. i bought it only to scavenge its meter to replace the non-working meter in my 680.
http://www.audiocircle.com/index.php?topic=67899.0