At present. I am using an Opera Consonance Cyber 20 (transformer-coupled EL-84 SE amplifier, outputting 1W into 32 Ohms) with these two headphones. I find it is a very good match for the HE-500s, only sometimes I wish for a brighter, faster, more incisive sound. With the HD600s, I enjoy the enormous soundstage and liquid, detailed midrange, but am distracted by a clearly audible 50Hz hum (presumably from the AC tube heater circuit). This hum is intrinsic to the amp's design - I've already returned the amp to the manufacturer and received the response "they all do that, Sir...". I have tried using my HD600s with the O2. This is not a bad combination, but loses out to the tube amp in terms of transparency, midrange detail and soundstage. It just sounds a little compressed and veiled. It doesn't really "come alive". Therefore, I am looking for a solid state or hybrid amp as a complement to the Cyber 20, to drive the HE-500s on faster-paced material and as a standard go-to amp for the HD600s (and the AT ATH-A900Xs, when I need some isolation....). So far, I am considering the Audio-gd SA-31 (probably the most cost-effective, as it is made locally in China, and there is no risk of being stung for import duty), the Lake People G109S, and the Project Sunrise 2 (for which I have a stash of suitable NOS ECC82 tubes in the basement - this is also the Cyber 20's driver tube!). I am, however, concerned that the Audio-gd may sound too warm and lacking in midrange transparency. Which of these amps is likely to be most suitable for my purposes?