I can only speak for the one I have, but I'm extremely happy with my Weber MiniMASS 50 - while you would need one of their larger attenuators for the JCM800, my MiniMASS is perfect for my lower wattage amp to keep the volume down when needed. I ordered my MiniMASS with the Headphone Tap for occasional extremely quite usage. It came with switchable 4,8,16 ohm output and treble compensation to bring back the natural reduction that occurs in the high frequencies ranges as attentuation is increased, and a line out so it can be used as a DI as well.
Mine looks like this:
They have the MASS attenuator for up to 100w - it looks like this:
Had several attenuators. Weber is great, perfect price to quality ratio. But speaking about the best attenuator EVER, for me it is the "Ultimate Attenuator". You can get a used unit for a decent price and it will literally punish any other brand, IMHO. However, it is powered and has a cooler, so it's not dead quiet like Weber (but you can fight that, and it won't be heard over an amp if you're practicing).