Do you like DIY? Bottlehead Crack OTL AMP (Kit/DIY) - Official Thread
might be something you’d like. Note that it might not be ideal for all headphones.
Other amps to consider might be the Atom or the Liquid Spark, if you want to keep to a tight budget.
Do you have any way to listen to other headphones to learn what you like? In the under $750 range, you will find lots of good headphones that sound DIFFERENT than the HD-650. I have the 6XX and a Hifiman HE-560 also a Grado RS1e. In some ways they are better, but in other ways not. I use all three, and an old pair of STAX SR-5n that I think are also the equal and different.
The Grados have better soundstage. The Hifiman has nice bass and a very pleasing presentation, and is fast. The Stax are exceedingly fast and detailed, but are light on bass.
I think in the sub $1k price range, you get better phones than most people have ever heard, but you are still choosing among compromises.
Having some choices make things less fatiguing. THERE! back on topic.