I have both C2A and Mainline. I would make this decision based on 2 factors.
1. What headphones do you have now and plan to obtain in the near future.
2. Do you want neutral, accurate, and detail retrieval that rivals any SS amp? Or do you want that OTL tube experience that is 3 dimensional and makes the hair on your arms and neck stand up and gets your heart rate going without even thinking about the minutia of how your gear is performing?
If it’s in your budget, I can almost guarantee you’ll end up with both, so this is more of “what to buy firstâ€.
I will say that my modded HD600/C2A combo and my HD800/Mainline combo are equal. Both of these combos are better than matching the HD800 with C2A or the HD600 with the Mainline. This is of course based on my music preferences. The 650 sounds amazing with the C2A also, I just really love this modded 600.
Since you already have a Crack I can see why this is a painful decision. If I had an upgraded Crack with film caps and WE 421A/6SN7 tubes (my good friend has this set up and I’ve heard it many times) then I’d skip the C2A and go for the Mainline.