I checked Elac's website....https://www.elac.com/authorized-online-retailers/
It does not look like underwoodhifi is listed as a authorized dealer....I wouldn't buy a 'fairly expensive' sub from a unauthorized dealer and be a sitting duck if Elac denies the warranty (Not concerned about the drivers, but the amps can be flaky as fck in subs no matter who the manufacturer is!)
Music Direct is definitely a listed authorized dealer and has them listed for the same discounted price. Might be a safer bet.