I had asked an amp builder/repair person that I respect and he pointed out that the horizontal nature of the tubes will cause the elements in the tube to sag and thus burn out faster.  In the horizontal position due to the heat generated the elements are not in a position to wear evenly.  This is not a direct translation from the person I spoke with.  This is my best recollection from a conversation I had with him.  This is not intended as a slam against DAR Amps.
I do agree that they look extremely cool and I was interested in purchasing one of them.