WOW! (which is mostly boroguru), is clean and potent but also one of the most expensive kavas you can buy.
Other's that are pretty similar in muscle relaxation are Mahakea, Stone, perhaps Hiwa/opihikao, and judging from the chemotype Malekula magic and pentecost should too.
It's typically the amout of kavalactone #5 and to a lesser degree #2, that contribute to muscle effects. They are longer lasting, more stomach upsetting and add a lot to the feeling of slowness & sedation and in higher doses, a bit of a loose doped up stupor.