RF Attenuators have been known about & used for many years in both audio & cable/satellite TV reception.
These particular attenuators are in-line adaptors that can be connected at the start or end (or both?) of your SPDIF cable. The idea behind using these attenuators on a digital cable is to reduce the strength of the signal. As a consequence the signal reflections are also reduced by twice the amount. Reflections are an inevitable occurrence that arise when a high speed signal meets an impedance blip on its passage from transmitter to receiver. These impedance issues could arise in the SPDIF transport, SPDIF connectors, SPDIF cables, or SPDIF receivers. These reflections are one of the many sources of added jitter in every digital system. Reducing the strength of these reflections should result in a reduction in the jitter generated by the transport to DAC connection. There are many situations where this should result in better sound, for instance, it may be a good way of reducing the sonic penalty usually incurred in using a BNC to RCA adaptor. So this could mean that by using these attenuators an RCA input on your DAC should now be about the same sonically as a BNC input.
To use these attenuators the reduced SPDIF signal has to be still capable of working in your system. This is certainly the case for the Hiface which has a high SPDIF signal to start with & may apply to other transports/receivers - real world results may be different although I have tested a BNC to RCA adaptor with & without one of these attenuators & the sound was noticeably smoother & without edge with the attenuator in-line.
Here is the cheapest place I have found on the internet for these attenuators at $12 each - there are other places that will charge you >$30 for eactly the same thing - I've even seen them for >€100 in RS which I thought must be a mistake! I believe this is cheap enough to allow experimenting with the range of values available. http://www.minicircuits.com/products/attenuators_coax_fixed.html
Scroll down to the bottom of the page - it's the BNC 75ohm variety that you want. They come in 3, 6, 10, 15, 20dB versions - the 3 & 20dB ones could probably be ruled out as too low & maybe too high an attenuation. What we want to aim for is enough attenuation to suppress signal reflections BUT not too much to reduce the signal voltage too low. You will know if you have done this as the DAC will not lock to the SPDIF signal.
Why buy expensive digital cables when using the high output Hiface & similar transports - these simple & cheap devices can be used to achieve a better solution. It's better solution because of the fact that a good digital cable if well made (expensive!) will have controlled impedance from end to end & will be a close match to the desired 75ohm. Outside of the cable at the DAC end or the transport end the cable cannot ameliorate any impedance anomalies that are encountered by the high speed signal so these will give rise to reflections travelling down the cable. The attenuators, attenuate these signals!