I'm also a Crack-to-Mainline guy. There is no question the Crack (both Speedballed and non) has a distinct character. It just sounds alive and musical, especially when paired with certain headphones. In my case it's the ZMF Auteurs and HD600s. The Mainline is just plain quiet, clean, tight, smooth and controlled. To me, however, it doesn't have a distinct sonic character. I like that it will drive just about anything and that it is conservatively designed. Neither of my amps is going anywhere. In fact, I sold my Cary HH-1 and Burson Soloist SL. I'm still searching, though, for that magical headphone pairing for the Mainline. I don't know yet what that is. Right now I'm using it primarily with Audeze LCD2Cs and the ZMF Auteurs. I've still got my eyes on the HD800, HD800S and (more recently) the Meze Empyrean.
In the end, both the Crack and Mainline are exceptional pieces of gear, which I'd recommend unreservedly to anyone interested in achieving great sound.
Best,
Joe.