Is a femto clock important if you aren't using asynch USB?

Some DACs like my RME adi2 always use their internal clock regardless of input type.   It's Femto clock is used also when there is a Coax or Tos. Input,  it doesn't just use it's clock for USB.

The better the internal clock, the lower the DAC's jitter rejection can work, to a point.  If upstream jitter is extreme enough no internal clock will fix it.

I agree though that chasing a 10 year old DAC seems imprudent. That was just after the cut-off range, when I remember Redbook playback getting much better.

I had a W4S Dac 2 (not DSD).  It was the worst sounding standalone DAC I have owned.  I would pass.

I have an old W4S dac2 and it is indeed a good dac if it works in your system.  Since you're shopping in this price range, I doubt you'll hear the difference with or without a femto clock  (this was a flagship dac over a decade ago but worth a couple hundred dollars today).

Be aware that the Dac2 is not driverless.  I got it work fine with my bluesound node but then Roon will not see it.

Jerry

femto clock is mainly useful to sync up the digital stream from a computer

Other than Asynchronous Rate Converter used in some DACs (like Benchmark), most of them use PLL (Phase Locked Loop)  and have variable D/A converter clocks with S/Pdif transmission, to adjust conversion rate to average incoming words rate (otherwise samples would be lost).  It used to be done with VCO (Voltage, Controlled Oscillator) adjusted by comparator comparing phase of incoming signal to phase of the VCO util they are even (same frequency).  Today it is likely all in digital domain, but still, D/A clock has to follow incoming signal rate, hence it is variable.  These "Femto Clocks"  are very stable, I'm sure, but average frequency stability is not important since we cannot detect such tiny clock differences/changes.  Jitter of the clock is important and likely audible when above about 50ps.  It is "picoseconds" - not femtoseconds, making me suspicious of snake oil.   With async USB samples come in packets (frames).  Each frame delivered at some fixed rate (like 1kHz) contains multiple samples.  DAC places them in the buffer and upon buffer over/underflow signals back to increase or decrease number of the samples in next frames.  That way D/A clock is fixed, but samples are not lost.  Unfortunately connecting computer also connects high frequency noise.  There are extremely low noise switching power supplies (SMPS) used by companies like Rowland or Benchmark, but those used in computers are cheap and extremely noisy.  This noise is finding any alternative return to ground, including one thru DAC grounds.  Optical isolation with USB would be my first choice of action.