The Mpow would technically work. Just remember that the HD600 are high-impedance headphones, so they won't get very loud with the weak amp that's in the Mpow. Whether or not that's acceptable is up to you.

To really get the most out of the HD600, you're going to want something with more power; preferably balanced. You're going to need a balanced cable and replace the Mpow with either a Fiio BTR5 or an EarStudio ES100. The balanced 2.5 mm output on these units provides plenty of power for the 300 ohm Sennheisers.

cable: https://www.amazon.com/NewFantasia-Balanced-Compatible-Sennheiser-Headphone/dp/B07VH4RTNG

BTR5: https://www.amazon.com/FiiO-BTR5-384K-Bluetooth5-0-Headphone-Unbalanced/dp/B07ZK3M6XK/ref=sr_1_2?dchild=1&keywords=fiio+btr5&qid=1591621450&s=electronics&sr=1-2

ES100: https://www.amazon.com/EarStudio-ES100-24bit-High-Resolution-Bluetooth-Unbalanced/dp/B078H4YD2L/ref=sr_1_1?dchild=1&keywords=ES100&qid=1591621496&s=electronics&sr=1-1

There are other options if you don't absolutely need to be wireless and/or balanced. The Topping NX4 is a dac/amp that you can attach to your phone with the included rubber bands and cable. It has plenty of power, but it's not balanced (so you can use your existing 3.5 mm cable). There's also the ifi Hipdac. It should have enough power unbalanced to run the HD600 just fine, but it also has a 4.4 mm balanced output, which would require a different cable.

NX4: https://www.amazon.com/Topping-Portable-Amplifier-Headphone-Compatible/dp/B01MS7TGB0/ref=sr_1_3?crid=1K5QIB194DHH3&dchild=1&keywords=topping+nx4&qid=1591622097&s=electronics&sprefix=topping+nx%2Celectronics%2C147&sr=1-3

Hip Dac: https://www.amazon.com/iFi-Hip-dac-Portable-Headphone-Balanced/dp/B0842W67F1/ref=sr_1_2?crid=46VQXH3GD82&dchild=1&keywords=ifi+hip+dac&qid=1591622160&s=electronics&sprefix=ifi+h%2Celectronics%2C154&sr=1-2

4.4 mm cable: https://www.amazon.com/NewFantasia-Balanced-Compatible-Sennheiser-Headphone/dp/B081PVCV27

My recommendation is to first try the Mpow. If it doesn't suit your needs, upgrade it to one of the 4 dacs I've mentioned using your existing 3.5 mm cable. If that still isn't quite enough, upgrade the cable to run balanced for the dacs that offer it.

Buy a DAC.