I've recently acquired my second pair of Sennheiser HD600s. The first pair I owned were purchased in the year 2000 and had passed their use by date. My new 600s are still being run-in, but sounding great.
My head amp is a Musical Fidelity XCAN V2, which was purchased at the same time as my original HD600s (year 2000), so it's getting on a bit. It still sounds OK, but I'm guessing that there have been significant advances in headphone amplifier design since the XCANs were released (mid 1990s?).
My budget is $1K to $1.5K. Valve or SS is OK. I've had a look (at least on the net) at the Woo Audio WA6 SE. A bit above my budget, but doable. https://www.addictedtoaudio.com.au/product/woo-audio-wa6-special-ed-headphone-amplifier
Any suggestions please?