At what impedances do Ampeg bass heads operate?

Here is a list bass head impedances:

SVT-CL/AV - 4 & 2 ohms
SVT-2PRO - 4 & 2 ohms
SVT-4PRO - 8, 4, & 2 ohms (stereo), 8 & 4 ohms (mono-bridged)
SVT-3PRO - 8 & 4 ohms
SVT-350H/AV - 8 & 4 ohms *V-4B - 8 & 4 ohms
B-5R - 8, 4, & 2 ohms
B-4R - 8, 4, & 2 ohms (stereo), 8 & 4 ohms (mono-bridged)
B-2R - 8 & 4 ohms

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