Oh then you have very good options. There are two types of DACs you may want to go for, since the THX789 can be used as a "single ended" and "balanced" amp, you can also get single ended, or balanced DACs. What balanced basically gives you is more noise rejection against something called ground loops. It's a bit too much to explain here, but it's mostly an issue in professional environments where electrical outlets are populated with a multitude of devices that can lead to a bunch of audible noise. You probably don't have this issue, but I want to give you the heads up if you do, going fully balanced (meaning your DAC + AMP + headphones) can give you protection against that stuff, and also usually an increase in overall power output (though the amount of power coming out of the 789 is already pretty off the charts, some people still go balanced due to the noise rejection inherent to the design).
If you're looking at the absolute highest performance for your buck, the Sabaj D5 is a DAC+AMP all in one unit, regularly can be found under $400, and can double as a second listening device if you don't want to use it as a DAC only (which you can, even though it has headphone amp functionality). This device is balanced, so theres a bonus with that if you ever decide to go balanced configuration in the future.
After that you have an easy pickup for around $200 (less sometimes like now $170 on Drop and places like Amazon), that is the SMSL SU 8. It's a pure DAC, and also has balanced. This would be a very simple device, and has been a staple for many folks wanting a balanced DAC that doesn't cost an arm and a leg.
Then for around the same price you have the Topping D50S. Essentially one of the highest performing DACs for its price range(though the Sabaj D5 is a tad better). It isn't a balanced DAC, but instead has Bluetooth input. What that allows is to pair your phone with it for example, and feed it audio from your phone to listen to on your headphones from your amp. Some people don't care for this feature. Me on the other hand wish all devices had it, so I don't have to turn on my computer or other source devices if I want to listen to music.
Another from Topping would be the DX3Pro. Also a single-ended only DAC, but like the Sabaj D5, it is also an all-in-one DAC+AMP. As a DAC it's great, and at a sub $200 price, another great bang for buck device. It doesn't have the performance of the D50S as previously mentioned, but it comes in at a much smaller footprint, and can be used as an all in one device as mentioned previously. Though low impedence headphones/IEMs aren't recommended as the Version 2 currently introduction has an output impedance that can change the sound of low impedance headphones/IEMs. But this isn't a problem if you use it as a DAC, only becomes a problem if you use it's headphone amp section. It also has that Bluetooth input as the D50S, so that's nice if you care for that.
You also have something called a Kadas Tone Board. A very small device that would require the purchase of a case if you opt for it. It's base price is $100, but as a newcomer, having an exposed board like that without a case isn't something I'd recommend, but nonetheless it has great performance.
And that's basically it. Keep in mind when I talk about performance of these devices, they're all already at such a high level of fidelity, I haven't seen anyone capable blind testing devices in this caliber and being able to tell one from the other in just pure sound quality. So either way you go, you'll mostly be making a purchase based on features, looks, and build quality. So any way you go, you'll be getting devices at the top of the performance charts. There are other DACs at even lower prices, who also are really good performance like the D30 from Topping which is a pure single ended DAC and comes in at under $100 that still performs great. All devices I spoke about now are pretty new (aside from the SMSL SU 8 which should be getting a big facelift and new internals sometime soon), so you can rest easy you're not getting some old bad performing stuff.