Both ESL variants work great in small rooms with some time spent experimenting.
Both designs are so far ahead of what often passes for high-end conventional 'speakers that even less than optimally positioned ESLs are usually preferable.
Iansr's ESL 57s will soon be mine. Yay! Here is a photo of wall-mounted ESL 57s - actually there are two more standing on the floor below the hanging ones: https://drive.google.com/file/d/1WEPgSYqlyaRj0BmbJkXn3JDASC12gH4M/view?usp=sharing
So it can be done. Sweet!
I also have experienced Magnepan 12 and 1.7i in my small room.
Expection bias? Noooooo way!
There are lots of 303s on the market in all sorts of conditions. So you should be able to get one if you want. Too bad you did not hang on to the STA 25 ... That is one I would like to hear some day.
I have had to have a rethink about my setup as we need to free up cash for my daughter's driving lessons and an upcoming Tenerife holiday . This means that I will swap my EAR 912 for an EAR 868PL and EAR 509s,so for a short while in January I will be able to compare the following set-up wirh my Quad ESL-57s (before one or two of the amps will have to go) :
EAR 868PL + EAR 869
EAR 868PL + EAR 509
EAR 869 alone (no phono stage)
It will be sad to see the Ear 912 go, but family is more important than hifi. And if I can keep the 868PL I will still have a very good pre.
Quad 57s should not be used on their side as others have already mentioned. Thery are almost square, so there would be no space saving even if you were to turn them sideways.
The rear of the speaker is covered with burlap, so rear radiation is attenuated, especially in the treble, and the speakers can be placed next to a side wall and close to fhe wall behind, provided that you angle them towards the listening seat.
If you value your speakers do not use them with an amp capable of putting out over 33 Volts peak, I.e. 66 V pk to pk.
High power amps will produce arcing which will damage the panels. Have the voltage clamps installed before you put the speakers into service. The Crown is a no-no. Stay with amps that are known to have good stability into capacitive loads and are rated at 50 W/8 Oms or less.
Unless it has been refurbished, the 57 should not be expected to sound at its best. So be prepared to have the panels and power supply checked and/or replaced.
Even factory spec 57s will not have a neutral balance in room, so be prepared to experiment with eq.
I will place the Quad ESL-57s on top of book-filled Ikea Kallax bookshelves angled down towards my listening position.