You are attenuating the mixer output by nearly 50 dB and then
amplifying up again. This is not good. I suppose you are
feeding into the low-impedance balanced input of the Carlsbro.
You would do better to feed into the high impedance unbalanced input

I opted for the low impedance mic input on the basis of advice in
rec.audio.pro where it was suggested to me to be wary of using the high
impedance instrument input as "sometimes these also contain fixed EQ to
sort-of simulate the tone shaping of a guitar amp; ie, a midrange
notch."

I therefore figured it was easier to just go with the mic inputs to
avoid this potential problem.

Is it electronic or electromechanical? If the latter, it will
certainly have been damaged. Sometimes they can be repaired.

Electronic. I'm guessing it's based around IC706 /707 on the power
board ( http://homepage.ntlworld.com/anengin..._Schematic.pdf ).
Does this look like a very basic delay circuit, or do you think it
should sound like a proper reverb ?