I would try a Cary, Prima Luna or Rogue tube preamp and I think you would get what you are looking for with your existing amps. That was my experience when I switched to a tube preamp.
Also, if you don't like the sterile sound I would for sure steer clear of the Benchmark DAC, it's as dry and sterile as they come. If you decide to go with an outboard DAC and you want warm I enthusiatically endorse the MHDT Labs Havana, great DAC. I tried over 10 different DAC's and I eventually ended up with the new Bryston, it's stellar.
Also, if you don't like the sterile sound I would for sure steer clear of the Benchmark DAC, it's as dry and sterile as they come. If you decide to go with an outboard DAC and you want warm I enthusiatically endorse the MHDT Labs Havana, great DAC. I tried over 10 different DAC's and I eventually ended up with the new Bryston, it's stellar.