To take this a bit further, if you are going to live with only one output tube, it would be optimal to have it be as neutral as possible, as this allows the sonic character of each driver tube to be revealed. And this is what makes the DRs very special as they do seem to be very neutral and transparent.
That said, the chance of a 6SN7, or any other double triode for that matter, being better than the DRs is likely very slim. But there is only one DR, whereas there are many, many 6SN7s. With so many to choose from, the selection of an output tube can be transformed into a rather different game, as the huge selection of 6SN7s offer the possibility of finding that one output tube which is the "perfect" match for a particular driver tube.
For example, the NU 6SN7GTs are typically quite bassy. Some have discovered that matching it with a 1940's Sylvania 6SN7GT, with it's thinner bass, but rather forward mids and highs is a great match. But for sure, rolling through many tubes in this manner take lots of time and money.
So in my opinion, to start, I would go for the most neutral 6SN7 I could find. And from my limited experience, I would start with a pair of Sylvania 6SN7GTAs or Tung-Sol "mouse ears".
Edited by gibosi - 3/7/14 at 2:23pm