Pioneer XA 780 Receiver sound !

My son has this vintage Pioneer 780 receiver and was using it with old JBL D 130 speakers
( in cabinets )... there was no problem at all. 
However he now changed speakers to vintage KLH 355 " baron " speakers... playing it at higher volume for TV as well as music.
Now the Pioneer has no audio but everything lights up. Could he have been over driving it ?
Opinions ...Thanks

Do you mean a Pioneer SX-780?  That is a late 70’s receiver.  It’s doubtful you could have overdriven the receiver without first hearing gross distortion.  The KLH Barron is an easy load to drive, so that shouldn’t be an issue, and Pioneer receivers have pretty robust amplifiers.

Obviously check all connections.  Plug a set of headphones in,  if they sound fine then it’s not the amp, your problem is a patch cord or connection someplace.

If it ends up being a blown amp, and a Pioneer SX-780, it’s worth repairing.

I'd check the fuses. IIRC there are some on a circuit board internally but its been years since I've serviced one of those.