I loved my Crackatwoa with HD600/800 cans, really great sounding amplifier. I have built half a dozen Cracks, some highly modded and it stood out above them all.
Not heard an actual Mainline, but have smilar amps with 6C45 tubes and found them to excel at detail, if you want the absolute best out of your HD800's with just a touch of tube sweetness, go that way. Technically superior, but it all comes down to a matter of taste at the end of the day. Vinyl vs CD, Coffee vs Tea, i would be very happy with either of them.