I have lower priced Grados and old Stax lamba's at home, so I can't help you there
If you are interested in headphone amps and have an interest in a great fully functioning tube preamp, try out the SOnic Frontiers Line 3, (or 2 or 1) it has the Headroom circuitry headphone amp stage and is one of the most neutral preamps on the planet. Great detail and neutrality in a fully functioning preamp. Check out the reviews on their website, the Soundsatge review included the Sennheiser interfacing with this headphone amp.
If you are interested in headphone amps and have an interest in a great fully functioning tube preamp, try out the SOnic Frontiers Line 3, (or 2 or 1) it has the Headroom circuitry headphone amp stage and is one of the most neutral preamps on the planet. Great detail and neutrality in a fully functioning preamp. Check out the reviews on their website, the Soundsatge review included the Sennheiser interfacing with this headphone amp.