I have hooked up my Pi with Hifiberry to a DIY preamp (tubes4hifi.com SP14) and poweramp VTA ST70 (also tubes4hifi). Great setup, great sound and a lot of fun putting it together in the first place. The Pi/Hifiberry also sounds great over a Bottlehead S.E.X. 2.1 amp.
Volumio is great. Thanks a lot to all the developers!