Ya i used my lc6i because I had a stock h/u in my honda. I checked my h/u through the wires that would go in to the lc6i, but they weren't hooked up to the lc6i when I checked for clipping on my h/u. What do you mean by how cracked up is the lc6i? If you mean how do I like it, I liked it way better than those high/low converters, my music sounded way better and the bass sounded cleaner but I have never tried anything else so I was happy with it, the dd-1 had a lot to do with my music sounding better also cause I was able to set my h/u and amps correctly.
If the only thing you wanna use the dd-1 for is to set up the lc6i then I wouldn't recommend it. And on my oem h/u in my accord it did tell me what volume it was at, it went all the way up to 40. But I don't have the accord anymore I got a tahoe now and I havnt put a system in it yet but I'm pretty sure imma use the lc6i with my stock h/u in my tahoe until I get an aftermarket one cause with the honda I liked my stock h/u but with the tahoe imma just replace it with an aftermarket one.