confused, 18v from distribution block fuse

So I put in a distro block that uses the glass AGM fuses (which I don't like too much) but I turned on my system and noticed that one of my JX1000 amps was not on. Tested all the connections, and it as very weird. On one of the fuse ends (outgoing to the amp) I got an 18v reading. I coming end was 12v. Took all fuses out, put them back in randomly and now a different amp didn't work. Replaced all fuses with new ones and eventing worked and tested 12v.
I have nothing that will produce 18v in the car, and the car was off.

-------------
2004 Hyundai Sonata
Kenwood KDC-X599 HU
2-Sundown X-18D4
2-SQ Q4500.1
2-SQ Q90.4
1-Massive DBX4
8-Niche 5.25 mids
8-Niche Tweeters
4-Skar 8" mid-bass
OhioGen 220a alt
OhioGen 350a alt
156.0

Your amplifiers produce plus and minus 20 some odd volts. But there is really no way that voltage can get our of the amp on the 12 volt wire.

Sounds like you had an open fuse but the amp somehow got power that operated internal circuits. (I now avoid my former beloved glass fuses... Manufacturing ain't what it used to be. Besides, clip contact becomes a problem.)

The main point being that most DMMs (eg, non "true RMS" types) give erroneous readings when (strange) AC voltages exist.
And since big amps use switched-mode power supplies, they inject - or rather, reflect - (AC) noise back onto their supply.
[ My 3-digit dash voltmeter shows ~12.7V when my 80W engine fan is on despite my system remaining at its usual 14.4V or 14.2V with over 460W of lighting on. Since the headlights don't dim with the fan on, the alternator's regulator is fine, hence inductive loads effect the dash voltmeter. ]


And tho it may sound impossible unlikely, the fuse may have formed a {contact) diode hence blocking your "18V DC ripple" - it would have been interesting to reverse the DMM leads to see if you got the same reading, or swap to an AC scale (and reversing that test as well).
The resistance of a bad fuse adds its peculiarities to the system.


It does illustrate one thing however...
Rotating all fuses as the first check for any electrical problems saves a lot of time, and posting on forums. Either the problem would have been (temporarily) solved else the cause obvious. No need to check wiring or test voltages etc.
(In practice, whether to rotate fuses or disconnect/reconnect all connections first is a thing of convenience - ie, do the easiest first; or maybe start other checks before difficult access fuses or connectors.)