I have not heard of the Crack, but had the 336 before and now the 339. I think the 336 is absolutely great with the Sennheiser, and it has endless tube choices as it uses the 6sn7. I listen to a fair bit of progressive rock and I never felt the combo being slow (that said, I normally reach out for an Alessandro when I listen to those genres).
If you read Headfonia's reviews, the Crack was slightly more preferred than the 336 due to their personal preference if I recall, but the 336 is cheaper and it is built while the Crack is a DIY. The 339 on the other hand is their favourite
LF 339... Auww That's out of my league, cost me $600 here... BTW, Grado considered as aggressive can right? Maybe it's a suitable match? How does DV 336 sound with HD600 and I wish you can share your tube rolling experience. Deep inside, I wish 336 tube rolling can outperform Crack or at least sit next to each other.
BTW, since you used to listened to both of 336 and 339, mind to share your impressions for these two? Oh my... I'd been spamming here and there asking question.