the 988's, 989's and 2805 are based upon the esl 63. consider the more recent versions of the quad speaker as modified esl 63s. they have greather extension, more dynamic range and they are better braced.
personally, i prefer the spectral balance of the quad 63.
however, having owned quad 57's for about 9 years, i would say they have the least colored midrange of any speaker i have heard.
you are a wise man to consider refurbished quad 57s.
i miss them. my concern with this speaker is the fragility of the panel. be careful of the amp you use, hopefully under 50 watts of tubes. do not play too loud.
personally, i prefer the spectral balance of the quad 63.
however, having owned quad 57's for about 9 years, i would say they have the least colored midrange of any speaker i have heard.
you are a wise man to consider refurbished quad 57s.
i miss them. my concern with this speaker is the fragility of the panel. be careful of the amp you use, hopefully under 50 watts of tubes. do not play too loud.