Think of the attenuator Ohm rating the same way you would of a speaker Ohm rating, because that's what the attenuator does...replaces the load on the amp.
Ideally, go Ohm for Ohm, the attenuators will work best that way...whatever the amp calls for, but you can go one notch higher...8 Ohm amp, 16 Ohm attenuator...etc.
HotPlates are each built for one target load rating, I've had one in the past. I currently have a bunch of the Weber MiniMass attenuators which focus more on amp wattage. That is, each MiniMass can work with 4/8/16 Ohms (there's a toggle switch)...but they come in different wattage ratings. I have a couple of 25W and 50W units. With them, if you plan on running your amp fairly hot/dimmed, you should get double the wattage capacity. 25W amp - 50W MiniMass. I really like them, they have the built in speaker motor that works very much like a speaker would...they are not purely resistive attenuators like other units.
I also have a Kendrick Powerglide, which is a resistive unit, it has an Ohm selector and can be used for variety of amps.
