Just sold a pair of Quad ESL-63 USA Monitors that I've owned for years. Really the best midrange I've ever heard anywhere, highs damn near perfect too. I know the 2805 is based on them but the ESL-63 did not have the midbass hump you described. The only think missing was that tactile lower bass you could really feel. The bass that was there was wonderful, but ultimately it didn't reach low enough or hit hard enough for me. But I've become much more obsessed with bass the longer I've been in this hobby, so my tastes tend in that direction (current speakers are SVS Ultra Towers which are bass monsters).
I've never heard 57s so I can't comment on those.
Issues you should consider if you go down this path is many ESL-63s/57s will require servicing, and both require plenty of room on all sides to sound their best. There are many people who refurbish old Quad speakers (I recommend Kent at Electrostatic Solutions, really great guy), but they are a pain in the ass to ship around the country (freight is most definitely required, so hundreds of dollars per speaker to ship one way).