The Benchmark is a great 2 channel preamp if the definition of great is not adding to the signal. The HPA4 was also excellent with my warmish Meze Empy headphones, both now sold.
The best rock and roll headphone I have heard is the new RAAL CA-1a ($2K). I have heard it only with the top end VM-1a tube amp, but it can also be connected to any of the headphone amps that are listed on this thread. The guys on Head-fi are saying it kills also with other amps, though the VM-1a is the king.
I read that the HPA4 sounded good with the CA-1a, though that surprises me a bit, must have been using a warmish DAC.