I have a pair of Usher cp-6381 loudspeakers but for further improved I filled the mass load chamber with 7.5 kg of sand and I replaced the original tweeter for the beryllium 9980-20 tweeter which is used in the dancer series. The Be tweeters were sold by the Dutch distributor. They even refunded the original tweeters.
I compared the cp-6381 + Be tweeters to the cp-8571 MkII and I noticed that de Dancer loudspeaker is a bit more spacious. It probably has to do with the design of the cabinet and the weight of the cp-8571 MkII. If you fill up the mass load chamber with sand the cp-6381 gets better. The midrange is more focussed and the bass has more control. With the cp-6381 + Be tweeters you will probably get 95% of the cp-8571 MkII performance. The sound of the Be tweeter is sweeter and more liquid. If you listen to the cp-8571 MkII you know what I mean. So far the US distributor will not sell the Be tweeters to the US Usher loudspeaker owners as an upgrade.