I've been reading that on the Mixer tab on the control panel 76% volume is the default for 0dB
On the FlexBass tab, is there any threshold value to the LFE Crossover Frequency?
Yes, it will resample to the sample rate you specify. Whether this at all audible is another matter. I keep it at 44.1.
The manual I've got says the exact opposite. The sliders operate on a percentage of full-scale, that is, 100 = 0dBFS.The Xonar driver has this weird behaviour where it thinks there might be clipping and then lowers the volume accordingly for a second. If you set the volume to 100% on every slider, the driver's limiter is pretty much guaranteed to activate, and your sound will become wobbly and have slightly reduced punch. My previous soundcard was a Realtek onboard that I blew up, which didn't have this behaviour. I never experienced any clipping distortion either, so for now I'm going to put this on the Xonar driver being overly cautious.The default settings, however, should be that the Left and Right slider in the mixer are at about 75%, and the main turning button on the interface at 50%. You should probably leave this as is, or experiment. Of course, if you're not hearing anything funny at all: don't worry about it!
Only if you're hearing distortion from your 2 satellite speakers. The slider exists because every speaker setup is different, so you should play around. It's possible that nothing needs to be done.
Where were you playing the Intermod Tests? I would expect that if you play them in your browser, they'd have issues because they're played through the MME engine, which on Windows 7 requires a patch (click) to avoid a resampling bug.