It is balanced or not?

After working on and rebuilding an XDA-2 DAC and looking at the XDA-1, my only answer would be "maybe?".  I see two 2134 op amps after the fully differential AD1955 DAC chip.  This suggests that the opa2134 op amps are working in fully balanced differential mode for the I/V section.  After that, it goes straight to the discrete analog output stage.  It is difficult to tell if this discrete analog stage is fully differential/balanced, but I would only assume so.

This is different than the XDA-2 dac, which has the same two opa2134 I/V op amps, but then goes through another single opa2134 op amp which acts as a low-pass filter and downgrades this to "single-ended".  Then the XLR outputs have their own set of opa2134 which convert the single-ended analog back to differential.  The XDA-2 is not "fully differential" from end-to-end, but the XDA-1 DAC might be.

@joser9616 - One point.   Just because they are "studio amplifiers" does not mean they are fully balanced.  The analog input stage of the amp may be balanced, but the amplifier itself is single-ended.  You can tell because the amp can be bridged-mono to double the power output.   You would not be able to do this on a balanced/differential amplifier.

In actuality, pretty much all studio equipment is not fully balanced.  "XLR balanced" was developed as a way to transfer analog signals between equipment devices, sometimes a very long distance (50-100+ feet).  In almost all cases I have seen, the XLR input on a studio equipment will always convert to single-ended analog internally for purposes of volume / equalizer / mixing / etc.  Then, on the output, it will convert back to "balanced XLR".

The op amp is a factor in sound quality, but it's never the full story.  You also need to look at power supply, etc.   The fact that the JBL uses a switching wal-wart power supply is an indicator of poor sound quality.  The JBL is meant as a cheap mixer / room correction unit.  I believe the Emotiva would have highly superior sound quality, especially since it has a fully discrete output stage.

yeah, that sounds like a good plan.  If you have two D-75 amps, use one for left speaker and use the other amp for right speaker.  That way, you are not sharing a single amp power supply for both speakers.

Running the sound through the JBL MSC1 is definitely going to degrade the sound quality of the Emotiva DAC, but if that's all you have, that's what you are gonna have to use. 

Is the amp better to use as stereo?  Generally speaking, yes.  It really depends on the speakers and if they drop below 8 ohms, but usually stereo amps do not perform as well in bridged mode.

Generally, a dedicated balanced/differential amp will have power from both the +(RED) and -(BLACK) speaker posts.  In this situation, you cannot bridge amp channels because they are already, in a sense, bridged. 

The service manual does not give a complete schematic, but if you look at page 6-3, you can see the Output Stage schematic.  On the right side of the schematic you can see the output connected to one speaker binding post.  The other speaker binding post is shunted directly to ground, so this is a single ended amp (even though the input stage is balanced).

Right.  If you have two D-75 amps, it is better to use one amp per speaker.  But not in bridged mode.  The power supply from one amp is only used to power one speaker.  This is better because a single amp power supply does not have to push two speakers.  You will get better results.

right.

No, not likely better than Emotiva DAC.