Wow. You'll get a lot of varied responses from this question, CW.
I use a Nautilus DMCIIB and a Fisher F75, both of which get well beyond the eight inches you mentioned. And both are set to disc out the stuff I don't care for, nails, etc.
But then my gf uses an MXT and I'm pretty certain an Explorer goes deeper than that, though I have not used one.
I like the Nautilus as a response to your question. I've used one for eight years now and have more than 3,000 hours on two units. I say that only to say I know the unit fairly well. When its disc is turned up to an extreme, you're still getting near its original depth as when you were using low disc.
But I would mention that this is in near perfect sandy soil we have on the SC coast. Very little (if any) mineralization.
I've perhaps 100 hours on the F75 and have found it appears to be the same in that you get very good depth even with a high disc number. I usually run about 10 on both the Nautilus and Fisher, come to think of it.
Hope this helps.
RIchard