Yes, but my brand new fully charged Energizer XP8000 or XP18000 could output 20.2 Volts (for loads up to 3 Amps) from its blue jack - which exceeds the Hi-Fi-M8 (and DACmini CX) maximum input voltage of 20.0 Volts (per Michael Goodman, above) by 1%.
This might be OK, but there's a safer alternative - use the green jack instead.
Now that my XP8000 is 18 months old, following many charge cycles, the blue jack is putting out 19.50 Volts, when fully charged - still more than the 19.30 Volts (measured) that the DACmini CX AC power supply (and that of the Hi-Fi-M8 ?) can produce - but well below Michael's specified maximum of 20V.
In my opinion, one should use the green (12-Volt) jack when powering a HiFi-M8 or DACmini CX from the Energizer XP8000 or XP18000, especially when the power pack is brand new. It will sound just as good with 12.6 Volts instead of 20.2 Volts, because, as Michael wrote, above:
When choosing an external battery pack, you have to make sure it supplies enough current (Amps) in addition to a DC voltage that fist the range Michael has specified.
The Anker Astro3 10,000 mAh LiPo battery pack, for example, which I've used with the Meier Stepdance and the iBasso PB2, gives you a choice of 5V (via a USB jack) and switchable 9V or 12V output via a barrel connector, but it's maximum current output is only 2.0 Amps vs. the XP8000 and XP18000's 3.0 Amps.
Going by Michael's spec for the HiFi-M8's switch-mode power supply, both the Hi-Fi-M8 and the DACmini CX power supplies provide 19V at 2.6 Amps. (My DACmini CX power supply is labeled 19V 2630mA.) Thus, at only 2.0 Amps, the Anker Astro 3 might not supply enough current for either the DACmini CX or the Hi-Fi-M8.
Here's a great article, dated 9 March 2011, from Tom's Hardware, reviewing several external battery packs. Don't miss the charts that show how they rated in terms of capacity (play time) per pound as well as other measurements. (Unfortunately, Tom's Hardware failed to test/confirm any of the rated output voltages.)