You could create a test CD with a pair of tones, say one at 60Hz and one at a multiple, like 300Hz. It should be easy to observe any time offset D vs A with a probe on SUB out and a probe on left or right. A tone centered on the x-over would be useful too, since it should be seen at both outputs. This should permit greater precision as to what the electronics are doing. There might even be a free 'scope software that is good enough if you're interested in trying it.