1. this first one may be due to the program you are using. I don't get that with windows media player but do with J River media center.
2. That click & pop is perfectly normal from these cards
3. High volume on low impedance phones with gain setting at its highest can damage the phones. Volume may even be sufficient to damage high impedance phones if left at high vomlume levels too long & may even damage your hearing if played at those volumes for any length of time. Many people here assume that the actual gain of the amp is being altered by the way. This is simply not true, the gain of the amp is fixed in hardware to +12db relative to the line out. The only thing that the gain selector does is limits the max volume by dropping the highest 1-3 bits digitally. Note that the 0db gain in the gain adjustment is actually 6db softer than the gain of the line out The only matching gain available from the headphone amp is through the front panel line out which incidently actually uses the headphone amp. I can tell by measuring the D.C. offset as well as there being no solinoid swiching sounds when switching through the various front panel gain settings or swiching from front panel headphone to front panel line out.
4. it is safe as long as you use reasonable volume levels that are safe for your hearing. Your settings seem good. I would not however use the bit perfect ASIO drivers if using the internal headphone amp to power your phones as any program that uses these drivers I.E. Foobar, J. River Media Center & such you will not have any volume control available & if headphone amp is set to the highest gain, I can just see it now, poof goes you headphones.
5. If just using Windows Media Player they are of no use. If using Foobar or J. River Media Center they do work. Wasapi prevents other programs from interfereing with the listener experience & some feel it offers better sound. It also retains the ability to control the volume. Bit perfect ASIO drivers ensure no alteration of the bits being sent to the soundcards DACs but disables any on board or software volume controls so you will need an external analog volume control to use this driver feature. Do not use the bit perfect ASIO drivers if using the on board headphone amp to power your phones directly as you will have no volume control.