shinsouAmping needs on the TH-X00 are minimal so I'd probably start with the Modi Mutlibit for my DAC if not getting the Grace m9XX and then pick up a nice neutral, smooth and detailed amp to drive them. Something with a nice dark floor and with whatever other features you might need/want.
shinsouThe Schiit (Modi/Magni 2 "stack", Jotunheim) and iFi products are very good and it's hard to go wrong with either brand. Just my opinion, but unless you have a very revealing playback system or superlative ears, I question whether or not you would really benefit from the Multibit version of the Modi 2 DAC. You'd save yourself at least $100 sticking with the Uber or regular 2, and it's something you could always upgrade in the future, be it Schiit or another manufacturer's DAC.
I'd also add the JDS Labs Objective2+ODAC amp/DAC to your list of considerations if you want an "all in one" box. There are lots of solid options to choose from. Cheers!
shinsouI concur with Schiit Modi/Magni or Jotunheim. Really depends on your price point, but even the base Magni is plenty fine. I personally use a Magni 2 Uber.
shinsouIf u are planning on purchasing other headphones in the future the Jotunheim will have lots of power for hard to drive headphones like a Sennheiser HD650. Keep in mind the Jotunheim is neutral/forward sounding so it isn't ideal for brighter headphones, I read up everywhere that owners of HD800s don't like this pairing.
It's actually the Magni that will power the headphones, Modi is strictly a DAC. That said, you are correct that the Magni is more than sufficient to push 300-ohm headphones like the HD650s. The Magni will drive just about anything up through 600-ohms pretty well (short of very inefficient cans like the HiFiMan HE-6). It's a fairly powerful little amp.
I don't have hands-on experience with the Jotunheim and it looks good on paper but, being an all-in-one box, you are limited with the DAC that's available for it. It only has USB input and, at present, you don't have the flexibility of swapping out a different DAC like you're able to with separate components. Not that there's anything wrong with the DAC in the Jotunheim, it uses a very good chip and perhaps you only need a USB interface; just pointing out the limitations with this device.
The FiiO devices are decent as well but, in my experience, they top out with headphones in the 250-300 ohm range. Anything with higher impedance than that and you'll require a more powerful amp.
Regardless of all this, the Fostex TH-X00 are low impedance and efficient headphones so any one of the amps mentioned in this thread, or even available here on MD, will easily drive them. It mostly comes down to how much the OP wants to spend and if they want to future-proof their playback system for any other headphones they 'might' purchase in the future. Cheers!
JTrubsThe op was willing to purchase a m9xx, the jotun is the same price with the dac and better in every way. I went from a fiio e09k + e07k --> Asgard 2 + Bifrost multibit --> Jotunheim + Bifrost multibit and maybe the Fostex's don't need a lot of power but they do benefit from a lower impedance amp. Never tried a magni 2 but the lower output impedance deos benefit and would be an ideal pairing. If the op purchases a magni 2 uber + modi multibit that puts him at 400$, 100$ less than what he originally wanted to purchase in the first place with better sq and in the future he wants a better amp he has the dac for it.
sirkuttin"too bassy" to your ears you forgot to add. To me this stack is very good and beats anything in it's price range including mojo that I used to own as well as sony ph-1a and oppo ph-2. I even sold my bifrost Multibit after I had Modi Multibit for some time and listned to these two dacs side by side. And it pairs very well with thx00 and th600 I own. It's also very good with IEMs due to low output impendance, zero hiss and compl black background.
DenonFanboyWhen it comes purely to output power the Jotunheim certainly gets the nod, but all other specs are a split between it and the m9XX; the Schiit is marginally better in some regards, the Grace is marginally better in some. The m9XX does have a better implementation of the AK4490 DAC and more functionality. That said, we're splitting hairs here as these measurements are in a realm that is not discernible to the human ear.
The Jotunheim, Magni 2, and Grace, among others, all have sub-1 ohm output impedance so I'm not sure where you're going with that in regards to the 25-ohm Fostex headphones? General rule of thumb is that you want an amp's output impedance at least 1/8th that of the headphone's impedance (some say 1/10th), so the damping factor is more than sufficient on these amps. Cheers!
DenonFanboyGotcha. And yes, if the OP has a budget in the $500 range, they would be wise to consider gear a little further up the food chain from FiiO. So much gear, so little time. Cheers!