The Quad ESL98 and ESL05 series speakers are near clones of the original ESL63. Electrostatic elements, audio transformers, electronics are identical. The frame of the ESL98 series is virtually the same as the ESL63 frame. The ESL05 series does have a much improved frame structure versus either the ESL63 or 98 series. The major difference is in the directivity achieved with the delay line. ESL98 and ESL05 series speakers have altered the frequency response pattern, increasing the high frequency energy of the speaker. While the ESL63 is essentially flat to 18kHz; both the ESL98 and ESL05 speakers have a rising top end. Build quality between all the speakers is essentially the same. The ESL63 electronics layout is modular in nature versus the single motherboard approach of the ESL98 and ESL05 speakers.