I'm looking forward ordering a Crack and building it. So it is best suited for high impedance headphones(HD600/650s). Are there any mods to make it play nice with Grados?
Yes, there are ways to make this amp run well with Grados. If you increase the current in the 6080, the output impedance will decrease. Along with this, the 100uf output coupling caps would need to be at least doubled to attain acceptable low frequency response.
The upgrade path we will offer with the kit will optimize it for use with 100+ ohm headphones, as 250+uf film capacitors are pretty tough to find, expensive, and physically huge. I would mention, however, that there will be some ways to modify the upgrade package, but I can't disclose much on that until the upgrade package is complete.