filter set up in esoteric sa-50

" I know it's down to personal taste "

You are absolutely right. Everybody is different. I clearly heard artifacts when upsampled Red Book or transfer it to DSD. Thus I kept it original. Filter I prefered was Short-Delay (may be less warm, ite true....but more authentic to acoustic music).

" i have hard time distinguishing the difference between x4 upsampling and DSD" If so then it does not matter, trow coin. I preferred to keep DSD as DSD.

When Alex Peychev from APL HiFi heard my unit he agreed with me 100% but installed his own DAC (insted of Esoteric) where everything was upsampled x4... (and output stage and his own clockd etc) and after very long break-in I love Alex work.

" I prefer up sampling CD's to DSD with my SA-50"

This is exactly what Stereophile reviewer done. He described every combination in smallest details except most trivial choice - keep CD sound original.

Everybody is different, I quess

I prefer up sampling CD's to DSD with my SA-50. There is mostly subtle differences but in my system I could finally discern some quick transients that I could never quite figure what the sounds were (two quick piano notes). To me it is more fluid and 3D, less glare. The FIR or S-DLY filters are not used when going to DSD.