Graham slee makes some very good headphone amps, you should check them out. I have a Graham slee novo and I’m very happy with it. It has a daisy chained output so that you do not lose an rca output on your preamp. This is great if you run a tape deck or mini disc recorder etc...I have run many a headphone through the novo and it performs great.
https://www.hifisystemcomponents.com/amplifiers/
https://www.hifisystemcomponents.com/amplifiers/