I had been using a profire 2626 for the past 12 years or so. I switched to the Behringer UMC1820 because it plays a little nicer with Linux.
The Behringer seems to have more low-mids in the output than the Profire 2626 did. It's a little off-putting to my ears. I was very accustomed to the old sound. I'm hesitant to call the Behringer muddy in comparison, but it might be.
1. Is it common for different output devices to have different frequency profiles? It sort of makes sense, but I didn't expect that.
2. What is a good way to determine what's neutral? (i.e. was my profire neutral, or is the UMC1820 more neutral?)
1. Yes- that’s art part of electronic design.
2. run a frequency generator and sweep the frequency at fixed amplitude. Measure the output signal vs the input signal at each frequency.
It looks like it might be in my speakers.
When playing out of my speakers, based on this tone generator there's an significant audible volume spike that starts at 125hz peaks around 145hz, and stops around 160hz.
There is no such spike in my headphones. The volume sounds pretty constant in those.
The speaker are a pair of Mackie MR5 which are ten years old now.
I don't believe this problem existed on the old interface. Any idea why it is manifesting now?
Too soon to say...
Your interface may have a bump....
or..... the Speakers have a boost or the room has modes at those frequency at your listening position
...or your headphones + plus your ears natural cavity resonance may have a dip
...do you have a scope or meter?
If You measure the electrical signal amplitude at the output (of both interfaces) while sweeping the frequencies you can eliminate the room and headphones influence on the measurement. This will tell you how much difference the interfaces are making.