I have a GNSC Reference modified Opus 21 and it was a signficant upgrade to my prior digital three-piece front end (CAL Delta transport, Genesis Digital Lens, TADAC DAC/pre). Overall beter resolution, more accurate bass and an awesome level of "musicality" (as others have noted here and in many other reviews). Jeff Kalt at RA is a pro, as is Steve Huntley at Great Northern Sound. I did have an issue with my transport, and Steve took care of it very quickly and inexpensively and with great care and communication. One of the great things about the Opus design is that the transport part of the unit is inexpensive and easily available. Jeff Kalt is now using a TEAC transport unit that he believes has solved a lot of the prior issues with the drives. One thing to note is that Steve at GNSC no longer accepts stock Opus 21's for upgrade due to a lack of available parts (no more Blackgates!), but he will still sell you a new one. While I have not heard a stock Opus, the GNSC Reference is great, and if you look around you can find a used one for only a few hundred dollars more than a used stock unit, even though the upgrade costs ~$1500-2000. Can't say enough good things about the Opus 21 GNSC Reference or Steve at GNSC.