I recently build a Bottlehead Crack and love the sound it makes. I've put about 100 hours on the stock tubes and from what I've read these are not the last word in tubes. I realize tube sounds can be very subjective and a matter of personal taste. I'm not looking for recommendations. I'm just curious what non-stock combinations users have used and enjoyed. The unit is driven with 12au7 and 6080 tubes.