Just curious whether this would make a difference. I currently run FLAC files off my hard drive through my Asux Xonar Essence sound card, via optical cable into my Pioneer 520VSX-K receiever. I'm considering upgrading to an
ME 240 integrated amplifier. If I was to do this, I wouldn't be able to use optical cable so would need to go back to RCA cable. Would this actually make a difference?
Maybe, maybe not. You'll be using the DAC in your sound card instead of the DAC in your receiver. Whether or not there's an audible difference is difficult to say. You'll have to listen to see
Also on this topic, I note that the ME 240 seems to have a pre out slot - I presume this means I can plug in a subwoofer? If so, and I want to adjust the levels the sub kicks in at etc, I am guessing I just adjust the sub manually (currently the receiver does everything)
You've eeb nspoiled by the AVR's bass management functions.
Most (not all) subs are designed to work without an AVR's bass management function so they can be marketed to stereo only users. And, unless your new amp is different than those I've seen, the sub's level will track the amp's volume control.
Many subwoofers include line-level inputs, which I KNOW your sub has. The sub also needs to have both a crossover adjustment and a level control. Neither of which are difficult to find on subs. But, some subs don't have these controls. If yours doesn't, you're in a bit of a pickle.
In a nutshell, you will adjust the crossover so the sub takes over where your main speakers start to drop off and the level to match as well. This can be done by ear but you can also get as fancy as you wish with test firmware and meters.
Once you have the sub dialed in with the crossover/level controls, when you adjust the amp's volume, the sub should be controlled as well by it.