I believe that, unless you listen above reference level, the DAC performance will be audibly the same as the 3600, 3700, or really any other modern, competently designed DAC. The 3400's DAC SINAD drops when delivering a very high voltage from the pre-out, so it also depends on which external amp you are using. If your amp can be driven to your normal listening level with around 1.5Volts or less, there is basically no measurable or audible difference in the DAC performance from the 3400 vs 36/3700. Furthermore, even if you were to use more than 1.5 volts which will measurably degrade SINAD, I doubt anyone would hear a difference in real life. There are lots of folks with old audiophile $10,000+ amps with 80 dB SINAD who think they sound great. IMO, the importance of ultra high SINAD is dramatically over exaggerated.
I doubt many/any could tell the difference(outside of their sighted imagination) between 80 dB SINAD vs 120 in normal, real use.