You have to first of all understand that Daniel Weiss is a well known and respected name in pro audio. Even though he has an “audiophile” line of products addressing the home user, the company is heavily invested and primarily focused on pro audio. Their 202 was a Minerva in different clothes.
And although I have never heard a 501, because I had a 202 I would like to share a few thoughts. Like some of the better pro audio DACs, 202 had all but the kitchen sink. Great preamplifier, amazingly transparent and detailed DAC, a headphone amplifier which was so good that I sold my not inexpensive Violectric straight away. Daniel continues to support his products with regular firmwares for years, sometimes adding significant functionality (the processing power is available under the hood). It absolutely wiped the floor with my second generation Benchmark, which sounded raw and unrefined by comparison. But it was still a Minerva and, while indeed an amazing monitoring tool with say an HD800, musical enjoyment was not its primary focus. And I would expect 501 to be much the same.
If I would need a fairly compact DAC for say a computer rig with an excellent USB interface, a great headphone amp etc - Weiss would be high on my list of candidates. For my main system I would look elsewhere, unless you have a valve preamplifier, a class A amp, laid back speakers (Sonos Faber, Harbeth etc). In that scenario I think a Weiss would work very well indeed.