Nobunaga,
The Purplehearts are quite low impedance headphones (25 ohms), so theoretically they will not provide an accurate sound, particularly in the bass, with headphone amps that have high output impedance. To match the Purplehearts you would want an amp with 3 ohms or less output impedance.
The Technics amp has an extremely high output impedance, which as Solderdude points out, means its output into your headphones should not be at all accurate. That does not mean you or others would not find it pleasant.
The Asus Xonar has a headphone output impedance of 10 ohms, so also too high to be accurate with your Purplehearts. Not a good choice for accurate reproduction.
The great majority of tube headphone amplifiers also have high output impedances, so will not match well with the Purplehearts for accurate reproduction. There are quite a few that will work well. The Woo WA6 is one example. Most are either Transformer Coupled (like the WA6) or are hybrid, with a tube preamp section and a solid state amp section.
So, neither the Asus or Technics are technically good choices...but, you like the Technics better. I think you need to compare to a better suited amp. Perhaps you should try or borrow a low impedance amp 1st (almost all of Amir's recent headphone amp reviews include a chart of measured output. See if you like an accurate amp low impedance amp 1st. If you do, then look for good matches like the WA6. If not, then you will probably have to listen to a bunch to see which ones best match your tastes. Nothing wrong with that