I've only run mine balanced; my amps are fully differential. Cables are Pure Note Titanium, 3m to each monoblock.
I switch between all the filters and find them quite useful.
I also did a long search for a preamp, but found that the P8i sounded better, in varying degrees, without one. The extremely low output impedance of the P8i means that it can drive any (yes, any) amp that has been even somewhat competently designed. The keys to good performance without a preamp are (1) low Zout source; (2) high Zin amp; (3) low capacitance cabling.
The P8i controls volume by digitally shaving off bits. This is why dCS tells you to keep the volume control above -20 dB. When the winter sets in, I plan to further modify the amps, installing ~6 position volume controls which will allow me to keep the volume setting on the P8i as high as possible.
I've also found that the PS Audio P500 is absolutely critical. This may be because I live in an apartment with noisy AC lines. I tried a few conditioners, but nothing was as good as completely regenerated AC; no real surprise there.
The P8i is finicky about the surface you place it on. They aren't kidding when they say, in the manual, to put it on a firm surface. I tried it on various suspension systems, and it always sounded broken. I'm currently using carbon fiber cones from Composite Products (available at Music Direct).
Admittedly haven't spent much time listening to it with the upsampling turned off. I tried it initially but found that I preferred the sound with the upsampling on. Maybe I'll give it a shot again.
Probably also going to modify the P8i when the warranty period expires. There are simple things to be done in the power supply that may make a nice difference (better filter caps and rectifiers).