How to switch the ohms on a crate GS-212V speaker cabinet

The most reasonable place for a new speaker is Seismic Audio https://www.seismicaudiospeakers.com. If you don't care for that, look for an Eminence Delta series. You'll need to check your amp for what ohm speaker you need.

According to the manual, it needs a 16 ohm minimum load, so no, you best not use an 8 ohm speaker.

On the back of your 6505 there are two jacks that are labeled speaker outputs. For mono function, use either one of those outputs and run a SPEAKER cable from your amp to one of the input jacks of your 1960a cabinet. I assume it is a 4X12 cabinet. Either input will work. Make sure you match the ohm selector on your 6505 (right next to the output jack, 4/8/or 16 ohms) to the ohm rating on your 1960 cabinet. It is likely labeled on the plate that houses the input jacks. You should be good to go. NOTE: Never turn your amp on unless it is hooked up to a speaker cabinet and never use instrument cable to hook your amp to your speaker cabinet. Bad for a nice tube amp like yours.
Rock on and hope it helps.

The Ohms for individual speakers is measuring the impedence of that speaker and NOT the pure resistance. Speakers are usually 4hom or 8 ohm impedence, but you can get outside those figures occassionally. When you talk of "Cab" are you talking of a "Speaker Cabinet"? What is it you're trying to achieve? Connecting the wrong impedence speaker to a device can do damage to that device or sometimes even to the speaker itself. May I suggest you say something like "I want to connect the speakers from my [Brand Name and Model] stereo sytem to my [Brand Name and Model] Ipod thingy." If this is the case, then you might do some damage to your equipment.

that depends on your cabinet(s). if you have an 8 ohm cabinet you want to use your 8 ohm out. if its a 16 ohm cabinet you want the 16 ohm out.. etc.. etc. etc.. owners manuals will tell you these specs

You best rewire the cabinet to put the two speakers in series. Wire one speaker plus to minus of the other and bring the other lead, one from each speaker to your jack on the cabinet. The amp output impedance is what is set on the switch... for two 8 ohm speakers you would use both jacks to the two speakers BUT ssince you have 4 ohm speakers the ONLY option is for two in series and set the switch to 8 ohms. The amp is ONLY 30 Watts so don't expect too much sound output... You should get a nice CLEAN sound at a reasonable level. If you drive the amp too hard it will clip and you will get garbage.

The cabinet is listed as a 4 ohm unit.

We would exoect to see four, 4 ohm speakers in series parallel or four 16 ohm speakers in parallel.

If teh unit seems to rattle, first put an audio oscillator into it and sweep the frequency. It is common for grilles and also logos attached to them to rattle.

Open the unit and power each speaker the same way individually for a test.

I suspect you will find either the grille or mounting hardware is actually doing the rattling OR possibly the wires inside are vibrating.

If the sound is a scratchy buzz, then the speaker(s) have probably had voicecoils damaged by overheating.

In this case, buy 4 new speakers and replace all. It is not likely to find replacements for the Buggera brand they put in these cabinets. Also the word on the web about that brand of speaker is not very good.

150 watts should be easily handled by four Peavey speakers in the connection you described.. If the Peavey's are 8 ohms, then your total load was 8 ohms for the series/parallel arrangement which is well within the 2 ohm drive capability of the Crate according to specs.

I question if you really mean the speakers were in series? This requires a special cable to do this. Now if you mean they were "daisy chained" like the amp went to one speaker and another cable connected that speaker to another, then you REALLY had ALL speakers in parallel electrically which would be at the 2 ohm low limit of the amp... In either case, with only 1/4 of the 150 Watts to each, the speakers should NOT have been damaged. The amp MAY have been damaged. Try each speaker individually at a REASONABLE level to test.

If you plug into the effects loop out jack,OFTEN the connection to the internal power amp is broken so you might have to arrange a special cable as a wye to go back into the power amp as well as your external amp. In any case pwere ALL interconnected amps, etc from the same power source/receptacle for system safety.

Use a SPEAKER RATED cable which will have good quality 1/4 inch jacks. Go from a speaker out on the head to one of the jacks provided on the speaker cabinet. NOTE: some newer speaker cabinets use SPEAKON connectors and if those are provided, they are more secure than 1/4 inch plugs/jacks.

PLEASE loop the speaker cables through a handle of the cabinet at each end to avoid them being yanked out while hot as the amplifier can be damaged if it happens.

OBSERVE the maximum load. If the amp says 4 ohms minimum, put no more than two 8 ohm cabinets on the amp as two 8 ohms parallel to 4 ohms.

DO NOT use an instrument cable.

if they are square cement power resistors they are probably 3-5 watt 300-500 ohm .... are there colored stripes on the resistors? those are probably grounding resistors, they usually are installed too close to the board, get real hot a bunch of times and then crack the solder welds. when you replace them bring up the height and make space under the resistor to keep it off of the board and put a little drop of silcone underneath to hold them in place.