First off those are OUTPUTS, not inputs. The "ohms" rating is what the cabinet is listed as. MOST (not all) Marshall 4x12 cabs are 16 ohms. You need to look at that little plate on the back of the cabinet and see what it says. It may say "ohms" or it may show an upside-down U after the #.
Failure to properly match the amp to the cab can COOK the output transformer,especially if the amp is cranked up.
As for the tone control settings, "death metal" usually requires that you turn DOWN the "Middle" control, crank the "Bass", and play with the "Treble". You basically want to "scoop" the mids out. Hope you have forgiving neighbors!

I'm talking about the inputs on the cab. There are two. One reads "4ohm input" and the other "16ohm input."

My neighbors will hate me forever!

I'm not up on that amplifier head. I would hope that it is a tube unit?

If you are running the single cabinet, choose 4 ohm, input to the cabinet. Select the 16 ohm input if you are going to use 4 speaker cabinets. 4 daisychained in parallel will equal 4 ohms. If you used the 4 ohm input with 4 cabinets that would virtually look like a short to the amplifier of 1 ohm and that could blow it up.

I am sure you have seen many metal guitarists that have 4 Marshall speaker cabinets stacked behind them. That's what it's all about to prevent shorting out the amplifier head. The amplifier probably delivers its greatest power output when loaded into 4 ohms.

Now if you want to try a slightly different sound. You switch the amplifier into 50 W mode. Now plug into the speaker cabinet 16 ohm input. You will be able to drive the amplifier harder since it will not deliver 50 W into 16 ohms. It will be more like delivering 12 W to the cabinet. So it definitely gives you a variety of things to try for different sound textures.

So there you go Mr. rock-and-roll man. Don't destroy your hearing anymore than it is already destroyed. Happy whaling! (I'm not talking about fishing or any kind of religious thing)

Remy Ann David

Yes, the 900 is a tube-head. outputs should me matched to input: 4:4/8:8/16:16 Ohms.

And don't scoop the f'in mids. You can try that later on when mixing.

I wouldn't scoop the mids, personally, because I play da blues. But I have recorded a couple of metal bands around here and the guys who have JCM900 Dual Reverbs DO scoop them. To each his own. I have always felt that with a Marshall tube amp of practically any vintage, the power is in that Middle control. But these kids these days, they don't see it that way. They crank the crap out of the amp and then scoop! Seems counter-productive to me...

Yup.. I play death metal myself, and to me it's all about mids. That's why I advice trying that technique on later in the mixing. Most would decide against it.

When the drums, bass, solos, rhytm guits and vox are in the mix, and once it hits a masteringstage, there will be enought scooping done to it to sound "cool".