The MOTU M4 is a high quality, yet relatively affordable USB audio interface with 2x XLR/TRS in, 2x TRS in, 2x TRS/RCA out, 2x TRS out, 1x headphone out, 1x MIDI in, 1x MIDI out. The M2 is very similar, just without the 2x TRS in and 2x TRS out. Judging by the firmware, I am tempted to claim that the M2 and M4 are technically identical, just with a different back panel with fewer connectors hooked up to the ADC/DAC.
Some high-end audio interfaces have a separate power input, but the M4 is exclusively bus-powered. In theory you only need the separate power supply when your computer does not supply enough power, but the M4 is well within the limits of what the specification guarantees. The M4 actually also has a standalone mode where you can use it as a mic preamp by connecting it to a USB power supply. However, like most audio interfaces, it has one power-related downside: it makes the speakers pop when it is turned off. Some people say it’s on the quieter end of the spectrum of audio interface popping, but I would rather have complete silence when I reboot or shut down my computer. Interestingly, MOTU managed to make it completely quiet when powering on. I tried a powered USB hub, but its power output is still controlled by the computer, so the interface loses power and the speakers pop when I reboot my computer.
There is a simple solution to this problem: the USB-C/PWR Splitter by 8086 Consultancy. You can use it to power the M4 from a USB power supply, but connect its data lines to your computer as usual. This solution may also work with some other USB interfaces, but it’s not guaranteed that the interface will keep its output powered when it loses the data connection to the computer.
There is also a significantly more expensive solution: the MOTU M6, which can optionally be powered by a separate but included power supply. You might also shop around for other audio interfaces with separate power connectors (Focusrite Scarlett 8i6 Gen3, Arturia MiniFuse 4, Universal Audio Volt 4), but it’s not guaranteed that all of them will keep running when they lose the data connection to the computer. I picked the MOTU M4 because its case and knobs are mostly metal. My previous audio interface had some parts made from soft-touch plastic, which after about a decade began getting stick and shedding drops of plasticizer, so I was not going to spend money again on soft-touch plastic like on the Arturia’s knobs.