Can a off the shelf DAC, if engineered/implemented good, sound as good as a custom DAC?

Look into the top tier SCHIIT DAC. You will be astonished. 

@axpert 

+1, I own the SCHIIT YGGY+ LIM DAC and like it very much    The LIM DAC setup was easy, though I do wish the input symbols were larger and easier to read.  I use one USB input from my Infigo Auto IS-1 Streamer (uses Audirvana Studio ) and one Toslink input from my LG OLED TV. 

The addition of the SCHIIT YGGY+ LIM DAC to my audio system has substantially improved my sound quality.   I immediately noticed my system sounds much better.  The music is clearer, bass is more powerful, and imaging has improved.  This is the best sounding DAC I have ever owned.   

The best advice I can give anyone shopping for a DAC is simple: listen to at least 3–4 DACs side by side and choose the one that engages your senses like no other. Specs, measurements, hype, and forum opinions can only take you so far. System synergy, room interaction, and personal taste matter far more in the long run.

In my experience, fewer but higher-quality components and a cleaner signal path usually outperform a chain full of tweaks and add-ons. A truly great DAC should draw you into the performance emotionally with natural tone, believable texture, effortless flow, and zero listening fatigue.

The “best” DAC is not necessarily the most detailed or expensive one; it’s the one that keeps you listening for hours and reconnecting with your music collection.

The sonic character of a DAC or any other component is the result of the entire component…. All the subcomponents, design, materials, case. Power supply is a big deal. Typically a very high quality DAC will weigh 35 or 45 pounds. Much of that is the power supply and vibration dampening material.

Long ago I owned three different DACs, all with ESS DACs. The sound from each was completely different. The most inexpensive was simply terrible, tinny and shrill. It’s not just the DAC… it’s everything.

Can a off the shelf DAC, if engineered/implemented good, sound as good as a custom DAC?

There is NO generalization as it depends on each implementation. A custom DAC can sound good or bad. Also, manufacturers may use the same chip which usually has a general sonic character, but their implementation and thus Sonics will be different.