DAC DIRECT IN TO AMPLIFIER OR TO PREAMPLIFIER

@ghdprentice 

You put together a system without one and you don’t know what you are missing. And this missing stuff is often the most important stuff…

Very true, both wrt to preamps and with other stuff too.  In my case it was dual subs, which I should have added years ago.

So, without a preamp you often get amazing detail but somewhat flat lifeless presentation. 

Not everyone has the same experience with loss of depth and dynamics, but it was mine with the 4 or 5 resistive passives I have tried in my system.  The internal volume control of the Metrum Adagio DAC was better sounding and certainly not flat or lifeless, but it leaned more toward a clean/detailed sound without providing the degree of body and rich tone I enjoy.  In addition to the Adagio, the only other passive solutions I have tried where that "somewhat flat lifeless presentation" didn't occur used Slagle autoformer volume controls (AVC) but those two units had their own sound that, while nice, I ultimately moved on from.

Regarding DAC volume controls and loss of bits, here is some reading material explaining how bits can be lost in some situations.  The article also includes a couple of illustrations by ESS showing how analog volume controls lower both the signal and the noise while digital volume controls can lower the signal while leaving the noise unaffected, which has the effect of reducing the S/N ratio.  

Not all DAC volume controls are the same.  The approach practiced by Cees Ruitenberg at Metrum and at Sonnet (now together under one roof!) involves changing the volume by varying the reference voltage of the converters, which, in conjunction with an algorithm they employ, preserves S/N.

In the end, as mentioned by others here, you really need to try things out for yourself and preferably in your own system with the music you listen to.

@lanx0003 

Although the average output impedance (OI) is rated at 300 and 600 ohms for SE and XLR line outputs, JA (Sterophile) pointed out that both SE and XLR OI can rise to 3150 and 3900 ohms at 20Hz. This necessitates an amplifier with at least a 40k input impedance to avoid bass sounding lean.

The case of output impedance rising in the lower frequencies was not unusual amongst tubed preamplifiers.  I believe it was related to the size of the coupling caps used.

With almost any tubed amplifier, and with most SS amplifiers, the rising output impedance wasn't a problem but there are some SS amps like my SMc modified monoblocks that have input impedance of only 10K ohms, or some of the earlier Pass Labs amps that had input impedances of 20K or 22K ohms, where a high'ish output impedance from a tubed preamplifier might roll off the bass early.  The rule of thumb was to have an input impedance of at least 10x the output impedance, but I have always preferred to see at least 20x.