Could I perhaps get people's more in-depth comparisons of the Auteur on the Crack+Speedball vs the Mainline? I'm debating between the Crack-a-two-a and The Mainline for mine and I'm unsure which amp would be the better pairing. On one hand the crack sounds like a cacophony of euphonic fun, but I like the sound of the Mainline's transparency, resolve and more open soundstage. Any head-to-head impressions would be greatly appreciated!! 
Happy to give you my impressions. I switch off between the two amps with the Auteurs. On the Crack, of course, I listen single ended. On the Mainline, balanced. Here are my thoughts:
- Crack/Speedball -- The Auteurs sound incisive, slightly more tilt toward treble (though not in a bad way) with a midrange that appears more forward to my ears. The low end is there and punchy. Good drive and pace. Solid and clean. Again, incisive, but not in any way etched. Good air and space: not dissimilar from the single ended 300B sound, with a little more control on the low end. Also reminds me of my Maggie 3.5Rs driven by the ARC VT200.
- Mainline -- The Auteurs sound more rounded and mellow here. Think ARC tube amp or Mac amp driving Spendors or B&Ws. Or, the B&W 801IIIs I had, driven by Class-A, Krell KMA160s., realistic. While there's a touch of warmth, cymbals, snares, higher register piano notes all sound very realistically rendered. Vocals, too, are realistic in their presentation. In general, there's more there there. Bottom end is solid and anchored. The mids sound right to me, though, depending on my mood and music selection, I may tend toward the Crack. This amp really controls the phones.
I hope this was in some way helpful. If not, I'll try to hone my vocabulary and give you something more to work with. In my opinion, you can't go wrong with either amp. I flop between the two. If I had to keep one set of headphones for both amps, the Auteurs would be them.
