The REF10 is really fine but at less than half the price, the REF6 is amazing too. I could see those who primarily listen to Rock and Roll possibly preferring it due to its more aggressive sonic signature.
There are currently some misconceptions out there that the REF6 has some technological advancements over the REF10. After researching it, I found this to be untrue. The REF6 technology is purely trickle down from the REF10. However, the REF6 does have material advancements over the REF5SE. To break it down, the REF5SE uses, per channel, one 6H30 tube for its input stage and one 6H30 tube for its output stage. The REF6 uses, per channel, two 6H30 tubes for its input stage and one 6H30 tube for its output stage. The REF 10 uses, per channel, two 6H30 tubes for its input stage and two 6H30 tubes for its output stage. Yes, the REF10 is essentially two REF5SE's, but the REF10SE or REF11, whenever it comes out, will not be two REF6's. This is because the REF6 bridges the gap between the REF5SE and REF10. The REF6 has improved coupling capacitors and power supply over the REF5SE but the REF10 already has these improvements or better. From my discussions with ARC, the REF10SE or REF11 is a long way off because they are still improving other components in their line from trickle down technology based on the REF10's design.
Best,
Ken