If these are the new KRELL Duo XD amps, then I can easily recommend the Benchmark LA4. I loved this pairing with the Duo 175XD. If these are the older KRELL amps then it may lack the warmth that some people like.
I may sell my LA4 to buy the Benchmark HPA4. It is identical to the LA4 but also with a good headphone amp. I have a great headphone in my future.
I never heard the Holo Serene preamp with the new XD amps, but I own both the LA4 and the Serene preamps and I know the Serene would also work nicely with the Duo XD line.