XLR out on a tuner?

Based on the Denon specs sheet, output voltage is the same regardless of the output selected.

Dan (Celander), where do you see that? What I see here is simply a reference to "output voltage 500mV / 2.2 kΩ." I would not infer from that that the balanced and unbalanced output voltages are the same; it is quite possible that the spec applies to one of the two outputs (probably the RCA output, given the 500 mv amplitude), with the voltage of the other output not being indicated. Also, while as you said balanced output voltages are commonly stated to be twice as great as unbalanced output voltages, with the balanced output voltages being implicitly defined as the instantaneous voltage difference between the two signals in a balanced signal pair, I’ve seen a number of instances over the years in which balanced output voltages (and also impedances) have been defined on the basis of each of the two signals in a balanced signal pair. As confirmed by John Atkinson’s measurements of some components that have been published in Stereophile.

Also, as I alluded to earlier, pro equipment is usually designed to accept and to supply balanced pairs of signals.

... if they used an op-amp on the XLR outputs to “split” the signal for balanced output, then each signal conductor would carry 1/2 the voltage.

No, that is not true. If an op amp is used to generate one of the two signals in a balanced signal pair by simply inverting the other signal, as is done in many designs, the "split" (and it is not really a split) would not result in anything more than a miniscule change in the amplitude of either signal.

Regards,

-- Al

What Denon did here is common practice in the industry, even now, and even in much more expensive components with high end pretensions.  Don't hate them for it.  In fact, some equipment has XLR inputs or outputs that are actually single-ended.  At least in the case of the gear you described, the XLR output is in fact balanced.  It's just not done in the best possible way.  And it may sound fine, for all we know.  Furthermore, Denon tells customers what they are getting, if you take the time to read the not so fine print.
I refer back to the Sony SCD1; yes, it had that suboptimally derived balanced output, but it was also regarded for many years as the best sounding one box CDP available and is still sought after by some collectors and tweakers. 

Al, 1+

Denon appears to adjust their output voltages as they seem fit. It’s 500mV for both outputs of the DN-300H (otherwise, they would have stated so if they differed). It’s different for their internet tuner, but they do specify output level differences where they exist. See:
https://denonpro.com/products/view3/dn-350ui

I got the tuner today. I know why someone sold it. It hums. Not through the speakers, through its cabinet, primarily the top. A small amount of pressure at a single point on the top stops the humming. I can only hear it with the music muted and I have to be pretty close. From my listening position I cannot hear it at all even with the music off.

It does not seem like a malfunction, I think it is just poor design and construction. Probably not enough for me to send it back.

Otherwise it is like new and works great. 

@n80. Weird.