Robonaut,
I don't know the Cambridge Audio 840A, but starting with a two channel setup and HT pass thru is the right path to follow. I would like to comment on your statement re. XLR inputs ("must have"), though.
XLR connections are most effective where the noise is high compared to the signal level. Since the background noise remains more or less the same with short cable lengths it is the signal level which determines the advantage of XLR over RCA. In other words, the smaller the signal, or the longer the signal path, the higher is the potential benefit of XLR connections.
Nowaday's digital sources output signals at very strong levels, often 2Vrms. That level is high enough to (over)drive many power amps. Consequently, pre-amps actually attenuate for the most part. The signal level on the connection between digital source and pre-amp/integrated amp is constant and high, therefore XLR connections are not better than RCAs per se. They do have an advantage between low level analog sources (such as MC pickups) and amp stage, and between pre- and power amps, if both ends of a connection are differential, XLR has no inherent benefit.
In other words, unless you have very low level sources and/or long cable lengths between pre- and power amp, don't worry about XLR inputs. Properly shielded RCAs will be just fine.