Sherwood Audio Players & Recorders - Page 3 - Recent Questions, Troubleshooting & Support

not sure if this link will help you out but have a look its all i could find at the moment . how to Factory reset Sherwood RD 7503 Google Search

If the amp has no subwoofer channel then you take the right and left channels to the speakers and feed them in the subwoofer first, then out to the left and right speakers.

surround and subwoofer may not work if there is an additional graphic equaliser operating.
1. check in your receiver speaker setting ensure your speaker setup doesn not have subwoofer set to NONE. this will block any signal to your subwoofer socket.
2 when you select your surround sound setting setup, ensure you select the speaker mode you want, if there is a multicontrol knob, cycle through the choices
select subwoofer mode and set it to yes.
if you have set-up small speakers S in your speaker settings then it could be that subwoofer is set to yes automatically if you have large speakers L, subwoofer has to be set additionally in the subwoofer setting.
3 test subwoofer on alternative device to test it is working ok
consult manual on enjoying surround sound and adjusting the speaker settings.

It certainly sounds like the main tuner has failed, if IC301 does that function I would replace it and see what happens.