Clank exists well below 18kHz. I have a low pass filter on my pedalboard. It usually sits somewhere between 3 and 4 kHz, and I get plenty of clank. Sure, some of it may be on the skirt of the filter (a bit above 4 kHz), but 18kHz bandwidth is far beyond what you need for a decent bass sound. I design pro audio gear for a living. Loudspeakers are horribly underspecified - the data you need to understand what a loudspeaker really does is well beyond what shows up in user manuals or advertisements. Unfortunately, giving more data (when customers don't know what it all means) doesn't help the situation - generally, more data brings more confusion. Use your ears - they're the final judge of whether or not the thing works for you, they should be the judge before purchasing as well. Yeah, I know you can't always do that.