I had the same problem dude, it took me an hour to figure it out. If you are still stuck give me a shout.

I encountered the exact same problem.  I even had three USB MIDI interfaces to try - AXON AX 50 USB, Kore 1, and a Novation ReMOTE 61 SL.  Line 6 Monkey wasn't able to find the DT50 with the first two.  It found it with the ReMOTE SL, but the firmware update resulted in the 8000000a error and rendered my brand-new DT50-112 into a light-flashing, relay-clicking, inoperable piece of junk.  Nice.  (In my seething disgust, I could only blame myself for breaking my solemn oath never to buy Line 6 again.)

I finally solved the problem using Monkey, but I'm not sure which steps made the difference:

1)  I replaced the USB cable between the ReMOTE SL and the Mac with a high-grade, high-speed, USB 2.0 cable equipped with monster ferrite beads at each end.  (Never had a problem with the existing cable before.)

2)  I replaced my MIDI cables with the highest-grade, best-condition cables I had on-hand.  (Heavy MIDI guy from way back, so I was fortunate to have a large supply.)

3)  I made sure of these MIDI connections:

  - DT50-112 MIDI Out <=> ReMOTE SL MIDI In

  - DT50-112 MIDI In <=> ReMote SL MIDI Out (not the MIDI Thru)

4)  Rebooted my Mac with the ReMOTE SL and the DT50-112 turned on.

5)  Ran no other software.

6)  Started Monkey, which found the DT50-112 only after I manually pointed it to the appropriate ReMOTE SL ports.

7)  Successfully updated the DT50-112 firmware.

Hopefully, the above will help those who are trapped in Error 8000000a hell.

I hate to say this, but I was really hoping my new POD HD500/DT50-112 experience would demonstrate that Line 6 management has evolved in the same posititive directon as their gear appears to be evolving.  But, unfortunately, that is not the case.

In it's typical, short-sighted, greedy, penny-pinching way, Line 6 saved a few buck by NOT installing a USB port on the DT25/50 series of amps.  And this, knowing full-well that the amps require firmware upgrades; knowing full-well that many of their customers don't have 5-pin MIDI/USB interfaces or cables; and knowing full-well that many of their customers who do have such interfaces will find them to be incompatible.

And, as if this isn't enough, the cheapskates don't even provide MIDI/USB capability with its POD HD family - which would allow their use for updating the DT25/50 amps.  How utterly cheap and shameful.

Line 6 has displayed this same brazen disrespect and disregard for their customers for years.  Management just can't seem to figure out that, in the long run, it costs them way more money than they save.  I wonder how many replacement amps Line 6 has had to issue; how many hours it has cost tech support;  how many people return their Line 6 gear and buy a different brand; how many of them never buy Line 6 again; and how many of them openly share their opinons and effectively stear others away from Line 6.  Do the math.

For now, I'll give this new HD500/DT50-112 setup my best shot.  It's for live use and it better kick butt.  Failing that, it'll be out'a here and I'll be on to an Axe-FXII or back to my racks, cabs, and pedalboard.