600ohms is old skool. Whether that means 150ohms is better for your modern new fangled bent? hmmm... i'm gunna find out about this

Well typically, 600 ohms was the line input and output standard and in all likelihood is referred to as + 4dbm into 600 ohm load. 150 ohms was generally reserved for microphone input loading, but in this case as an output, it can deliver greater current output than the 600 ohm output. Even if your meter is reading 0VU, you'll have a different output level depending on which impedance you select. You also have a different frequency response whether you terminate the output or don't terminate the output. If you terminate with a 600 ohm resistor, you will hear a different frequency response that in all likelihood will be flatter than if you don't put a termination resistor on the output. Likewise for the 150 ohm output.

Now you're Mbox probably has a 10,000 ohm input impedance, that's OK. It certainly will not load down the old Altec. Just go ahead and experiment with both outputs, with and without load resisters. You may find that you like recording vocals with the 600 ohm output, without termination as it will increase high frequency response. Conversely you may want to cut tracks on piano with the 150 ohm output and its associated matching load resistor for a more mellow tone, etc..

It should be a very sweet sounding front end for your input purposes. Good choice!

Ms. Remy Ann David