Hot Plate

There is only one THD Hot Plate that will work with the 5150. The 8 ohm, purple THD Hot Plate.
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If you’re using the HotPlate as a load box only w/o a cab, it doesn’t matter which one: 4, 8 or 16. Just set the amp’s ohmage accordingly.

If you’re also going to use it as a vol/power attenuator, get one that’s rated for your cab.
i.e. I have an 8 ohm cab, so I have the purple 8ohm HP.

The Weber’s ohmage is variable/selectable, but make sure you get one with enough wattage to support your amp.
The Megadump is rated for 200W, but they have others that are 50w, 100w, etc - with different options on them and priced accordingly.

Hot Plates are rated for 150 watt continuous, 185w max and have noise reduction circuitry.
 
why do you state only an 8 ohm will work?

4,8,16 will work and selecting the appropriate 5150 load setting.

True, I should have clarified that I meant that there's only one Hot Plate, and that's THD. There are other attenuators on the market, but they are not called "Hot Plates," that's THD's trademark.

I've tended to find that 5150s run best in mono at 8 ohms though...