difference in levels between different AD/DA converters

[subatomic]

maybe this is a better forum for this question. I'm getting no help anywhere else.

I recently started using a Fireface. I'm using a Tango24 on one of the adat i/o.
My problem is that with both units set to +4dBu reference level, and a test tone sent to both an analog input on the Fireface and on the Tango, The Fireface input meters show the Fireface signal to be approx. 6dB hotter than the Tango input.
On output, I get the reverse. Same signal sent to Fireface and Tango outputs. Tango output is approx. 6dB hotter than Fireface output.

I dragged out the manual for the Tango 24 converters. I thought that I'd compare the specs of the two units' analog sections.

here's what I found:

A/D
Tango lists "headroom" at 15dB. (maximum level 5dBV at -10dBV reference level, or 19dBu at +4dBu reference level)

Fireface shows maximum level (0 dBFS) @ Lo Gain: +19dBu, @ +4dBu: +13dBu, @ -10dBV: +2dbV

D/A

Tango lists "headroom" at 15dB. (maximum level 5dBV at -10dBV reference level, or 19dBu at +4dBu reference level)

Fireface shows maximum level (0 dBFS) @ Hi Gain: +19dBu, @ +4dBu: +13dBu, @ -10dBV: +2dbV


it definitely looks like the Hi Gain and Lo Gain specs from the Fireface match up with the +4dBu specs on the Tango. That would be consistent with what I've found in testing. With Fireface at +4dBu, I observed an approximate 6dB difference between the Fireface and Tango. This would seem to be consistent with the specs.

It seems that if I operate the Fireface at Hi Gain output and Lo Gain input, that the levels (and specs) match the Tango 24. Is there any drawback to operating at these levels? The manual isn't exactly clear what those levels are even for.

I don't believe that either converter has options for calibrating input and output levels. Maybe this is a sign that this doesn't belong in High End?

Can anybody help?