Have a look at this Barry, solid state rectification for IMA series but I’m pretty sure these are K25 amps. You can just see the XLR input in the EBay photographs. It seems the K25 were a modified MA-25 for the BBC specifically for use with the KEF LS5/1A monitors. They amps have a filter for evening out the speaker’s frequency response but this can be switched out. They also have input trim pots but no impedance selector switch.
https://i.imgur.com/hp68Sld.jpg
https://i.imgur.com/qC0EzBM.jpg