Good discovery!
With the sub disconnected is it merely "better," or is it totally cured, with no distortion and balanced channel levels?
Is the subwoofer something you added recently which caused the change in SQ or has it always been there?
If you are using the SP-8's tape output for the sub, then does the Monitor/Input switch position affect the problem? Maybe the tape amplifier is not on unless the switch is up. The tape output and the main output have separate cables, it is not the same signal, so there could be a tape buffer (current amplifier) circuit, which if not engaged or if broken could cause this.
The cable length will not affect the load significantly. If the cable is healthy it should not be an issue.