Nuforce amplifier is designed with a true-differential input topology, whether connected via RCA or XLR, the input is setup to provide the highest common-mode noise rejection. So, performance wise, there is no difference, as far as Nuforce power amplifier is concerned. However, for system performance, it depends on your upstream component.
If your preamp is an XLR, fully balanced design, then obviously you would want to use XLR cable.
Beware that most dual RCA/XLR preamp are either (1) single-ended with XLR connector (2) single-ended with posteriorly patched phase inversion circuit. With type (1), no difference. With type (2) - probably worse using the XLR. Plus, in a mixed RCA and XLR system, the chance of having a ground loop is greatly increased due to complication in system grounding topology.
Our opinion: in almost all cases, we are dealing with single ended sources, XLR offers no obvious benefit other than added cost and potentially corrupting a perfectly fine signal. Since Nuforce amplifier is agnostic to RCA or XLR, for cost and performance optimization, we develop our P8 and P9 to provide optimal performance even they are single ended design - we simply believe our preamps outperform competing products that offer type (2) XLR configuration.
