Not sure if all tube amps will mellow the sound.
YMMV (and can be disputed) so I say with quiet voice, but I would not use tubes for small signal amplification, even if it's easier to build good sounding amps with them: IMHO MOSFET or LD-MOS can be better.
For start, try any of these:
JDS Labs Atom, Schiit Magni (IMHO better than Valhalla), JDS O2, Topping DX7, Pro-Ject Pre S2 Digital, Metrum (e.g. Amethyst), RME ADI-2, Matrix Audio products, etc.
Also take a look at others' lists, for instance https://www.themasterswitch.com/best-headphone-amps
Disclaimer, I don't use any of these any more. For dynamic headphones I use a custom built amp with Panasonic LDMOS (not commercially available).
IMHO the slight brightness of the D9200 mainly comes from the stock cables. I tried 4 types of copper cables with the D9200, brightness was gone with all of them (even though there've been sound differences).
Trade the D9200 cables with D7200 cables and both parties will benefit . Or use any copper cable (except the short D9200 cable which is quite bad).
YMMV (and can be disputed) so I say with quiet voice, but I would not use tubes for small signal amplification, even if it's easier to build good sounding amps with them: IMHO MOSFET or LD-MOS can be better.
For start, try any of these:
JDS Labs Atom, Schiit Magni (IMHO better than Valhalla), JDS O2, Topping DX7, Pro-Ject Pre S2 Digital, Metrum (e.g. Amethyst), RME ADI-2, Matrix Audio products, etc.
Also take a look at others' lists, for instance https://www.themasterswitch.com/best-headphone-amps
Disclaimer, I don't use any of these any more. For dynamic headphones I use a custom built amp with Panasonic LDMOS (not commercially available).
IMHO the slight brightness of the D9200 mainly comes from the stock cables. I tried 4 types of copper cables with the D9200, brightness was gone with all of them (even though there've been sound differences).
Trade the D9200 cables with D7200 cables and both parties will benefit . Or use any copper cable (except the short D9200 cable which is quite bad).