I'm using a Schiit Asgard (version 1) that I got second hand for a comically cheap price. It's in mint condition. The guy had upgraded to the Lyr and just wanted to unload this thing.
How much of a difference does it make? Well, you can't compare apples to oranges. The Fulla is a low end product. So is the Asgard but it is not as low end. Basically, I went from having decent headphones to having a nightclub inside my head. If you blind folded me and asked me what the difference was between my integrated amp's out and the Asgard, I'd tell you that you had given me headphones that cost twice as much as the 400i's when it was hooked up to the Asgard. I'm listening to it right now. It's pretty freakin mind blowing. I would not hesitate to buy another Schiit Product...
I listened to another headphone amp from Hifiman that cost more than the Asgard does (in new prices) and the Asgard destroys it in the worst way. No comparison. And the Hifiman was a tube amp too.
You won't get a mind blowing performance out of the Fulla but it'll be decent. I wonder why you spent $500 on your headphones but less than $100 on your amp/dac? Would have made more sense to pick up HD598's and a Schiit Magni 2 modi 2 combo. JM2C.