For me Crack is all about the journey so upgrading to another product before it's over would defy that point.
Starting with Crack and upgrading it step by step teaches you how certain components influence the sound.
It shows you what makes sense and what upgrades don't.
From a financial point of view it's most economical to don't upgrade it at all or to start with a Mainline and be done with it
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The small footprint does really make Crack great. It fits even the smallest desks. I move once every two months on average and it easily fits in one small box (with tubes, DAC, headphones and cables).
In small bedrooms i've also had it sitting on a nightstand (you know, mini hotel size nightstands or on a chair if needed).
Mainline top plate is 12" by 12" or something. Crack is 10" by 6".