A lot of Naim pre-amps can't cope with how loud DAVE is in DAC mode. DAVE's pre-amp mode sounds identical in terms of quality. Find the sweet spot for the volume so that the Naim gear doesn't get upset.
You've probably discovered that radio frequency noise is an issue with DAVE. So I recommend you first try to understand how DAVE should sound, even if only temporarily using a crazy setup. So find a device that outputs optical. Make sure no other inputs on DAVE are connected. Or, if you have a phone, tablet or laptop running off battery power, USB from this device is a good substitute for optical (again, with no other input cables connected to DAVE). The key here is to determine the true potential of DAVE.
Use this setup as your reference for sound quality when trying to get your server to sound its best. If you find the server sounds worse, I suggest 30 or 40 clip on ferrites added to the DAVE end of the USB cable. You can spend thousands on USB cables and they cannot be as good because their RF noise filtering is too weak. The galvanic isolation on DAVE's USB input still lets through too much RF noise, sadly.
RF noise getting into DAVE is the sole reason for variations in its sound quality. DAVE isn't affected by jitter.
In my signature there's a link about power cables. It's a simple improvement and in the end the difference is fairly subtle. Your Naim gear will probably object to RF noise in the power cables more than DAVE will, and DAVE may be sending RF noise to your Naim gear. So it may turn out that treating the power cables for your Naim gear is more important than for DAVE itself.
See if your dealer can lend you a Chord Etude to put between DAVE and your speakers. I think this is the next thing you should try rather than M Scaler (it's about £500 more). If you're using NAC-A5 you'll probably want something else while trying Etude. Etude's design prefers XLR cables to RCA. DAVE + Etude is infectiously beautiful
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