I have recently received my Reference 9s back from a couple of final mods from NuForce. My amps are the version 9.02 and have extended bass, extended bandwidth, and the gold/copper WBT NextGen RCA inputs. Before this last mod, I also discovered that removing the yellow capacitor across the speaker outputs extended the bass and made the sound more transparent. So mine are without that output capacitor, as well. This is currently the latest version, and can be bought from NuForce with all the upgrades I have.
I think part of the reason I like these amps so much is because of the unique super-fast digital power supply that NuForce has developed. It seems to enable high dynamic capability with high speed (to catch the leading edge of musical transients without smearing or blurring them).
One poster said he thought the amps sounded more like what he hears live than any other amps he's tried and I agree with that. I use the super-fast VMPS RM30 speakers and can tell you that not all speakers are capable of responding accurately to the speed of the signal that the NuForce amps can deliver. The VMPS ribbon drivers are incredibly quick and detailed and with the latest Reference 9s will provide a near-live listening experience if the recording and the rest of the component chain is up to it.
Regarding tonality, as has been noted here, there have been different versions of this amp and different combinations of upgrades that have produced different results in my system. I liked the NextGen Platinum RCA inputs with earlier versions of the amps, but found them too bright for this latest, extended bandwidth version and had them changed to the copper/gold version, which has worked well to provide a natural tonal balance.
Another note on tonality is that the NuForce amps are very cable sensitive (just like most amps). They are especially power-cord sensitive and if you find the right combination of interconnects, speaker cables, and power cords, they will provide very natural and transparent sound.
The bottom line is that these latest Reference 9s, the 9.02 upgraded version, have taken my system to new heights that I believe would be difficult or impossible for other amps to match.
I'll be writing a full review for www.stereotimes.com, shortly.
Happy Listening!