Hi I have a PANASONIC RE-7176 in beautiful condition. It has PHONO, REC, and 16 OHM outputs on the back. For some reason, I can't get anything out of these ports but crackling and a hum in the 16 ohm

SOURCE: 4 ohm and 8 ohm speakers

You can plug in higher ohm speakers , the higher the ohm the higher the resistance is, it is a danger when you plug lower ohm speakers into a higer ohm Amp , at a high volume that will cause them to blow , the center speaker would be fine to use a higer ohm speaker , your best choice is to buy a Active Subwoofer, which means the sub has it's own power supply , and u can blast the thing as much as u like , thn u can turn the bass down on all the other speaker's so u can play it louder , and have the sub turned up has high as u like , this is the best way to get great sound with high volume , buy a Active sub woofer , any active subwoofer is ok , a active sub woofer has its own volume and inputs on the speaker ,

SOURCE: 6 ohm speakers - 8 ohm amp

This is a tough one due to the info at hand, but I'll take a stab at it.
I'll assume that your amp is not a tube-type amp. I'll also assume it is a relatively recent "solid state" design. If those assumptions are correct, connecting a 6 ohm load to an 8 ohm amp should not cause significant overheating or amplifier instability.

SOURCE: 4 ohm speakers for 8 ohm impedence on receiver (HT)

ya cant run them 4 ohm on 8 ohm sterio run super hot then fail sooner gota get sum nice 8 ohm ones ok flouece off ebay is relly good cheap 8 ohm

SOURCE: no output to speakers

Yes inside should be 2 for the speakers and phen a power fuse

SOURCE: Marshall jcm 2000 dsl 100 head not powering cabs 4/8ohm outs

Since you are getting some output, the amp section is probably OK. There are circuits designed to handle thge difference impedence outputs. This should be a relatively simple repair for any shop that handles these units. If you don't have one local to you, let me know and I'll locate the service info and get you more specifics.

Dan