Heya,
Hold the phone, friend. Do not spend a dollar right now.
The AT-HA26D cannot drive the HE-4. That thing is just not going to give you even close to enough power.
The Matrix M-Stage is a good amplifier, but it's still not powerful enough for the HE-4. The M-stage doesn't even push 1 watt. The HE-4 needs about 2.5 watts at 38ohms. This is speaker amp territory.
Not to sound patronizing but you really should have researched the HE-4 a lot before you acquired it. Maybe you got a good deal on it and lept on impulse. You can share that story if you wish. But to drive the HE-4 properly, it's an 86db/mw inefficient headphone, this is like a speaker more than a headphone, it needs a lot of current throughput to drive from an amplifier, on top of being 38ohms, combined it takes a ton of energy, hence me saying 2.5 watts at 38ohms which most headphone amps simply cannot do (some can). The M-Stage does not produce this much.
So before you drop more money and literally waste it, please, stop and take a moment to know what you need for the HE-4. The HE-4 and the HE-6 are two very unique headphones in the sense that they are more like speakers in terms of how much power they need due to their incredibly inefficient design (HE-6 being even harder to drive at 83db/mw). The HE-4 is 86db/mw.
If you want an all-in-one solution for the HE-4, the best option for the cash is going to be the Audio GD NFB 12 (used), or 11.32 or 15.32 used/new. They will run you $150~250. But they have a good DAC, and the amplifier will hand out over 2 watts at 38ohms which is what is needed for the HE-4.
Very best,