quad esl 63 needs a sub woofer

I have an quad esl 63 and I am planning to add a sub woofer. I need help about choosing one. I heard about gradient sw 63s. have anyone know anything about that. I am using tube amplifiers since I will need an extra amplifier to drive the sub, do you think eloctrocompaniet ampli (Class A)they match with the tube amplifiers. Thank you. Note:they are selling gradient sw 63pair( excellent cond.) for $1900. Do you think I should go for them?