Just my 2c (others may have differing views)
- For a computer based system, you may as well go lossless - as disk space generally isn't an issue. The reason I suggest this is simple - if you have lossless, and have to transcode to another format - you're best doing it from the original. Your call on lossless preference. Personally I use FLAC because the tagging is pretty good. BTW - don't try and transcode your lossy back up to lossless (ie change your MP3 to WAV/FLAC) - once the info's gone you can't get it back again (hence the reason it's called lossy).
- WAV is not 'best'. It's lossless - just like ALAC is lossless and FLAC is lossless. WAV is just not compressed. Honestly - rip to ALAC if you're in an Apple eco system, or FLAC if it's Windows/Linux. WAV has no sonic benefits - just a larger size. When your PC decompresses the FLAC file - it outputs exactly the same PCM stream (as a WAV file) to the DAC.
- I haven't owned the STX (I have an x-fi) - although I did own and enjoy the AD700. Just be aware that the output impedance on the STX is ~ 10 ohms. So it isn't an ideal damping factor with the AD700 (32 ohms) - ideal is about 1:8 or more. However saying that, I've heard of many people pairing the two together - and haven't heard any critiques. The STX are supposed to be good cards - and if you move up to something like a Q701 or HD600 later - the damping factor will suit better.
- If you're just going to use the STX (it should be good for movies, music and gaming), then I'd consider connecting the speakers to the rear RCA analogue ports, and getting a 6.3-3.5mm extension from the rear headphone out port and just loop it over the top of your desk. When you want to use your speakers - turn them on. When you want to use the AD700s, turn off the speakers, and plug the headphones into the extension. Another alternative would be to always have the headphones plugged into the back port via the 6.3mm plug. Final option would be to make sure the STX headers are correctly connected to the front ports of the PC, disable mobo sound, and plug AD700 into front ports (it should then be using the STX). Only issue with that could be shielding - again I don't know enough about the STX - so maybe better to talk to an STX owner.
- If you're going to get the STX - my advice would not be to get a stand alone dac/amp - as the STX actually has a really good chipset & really you'd be paying far too much and not using the features it provides.
Hopefully none of that was too technical.
If I was making the decision - here's what I'd consider:
[a] How important is gaming - and I mean specifically directional queues (ie FPS)?
[b] How important is music? Does it have more importance than gaming?
[c] How likely are you to upgrade - especially headphones?
If gaming is quite important - then either STX, or lower model Xonar which has the dsp features + an amp. PurpleAngel might be able to help you with suggestions.
If gaming and music equally important - then STX alone.
If gaming not so important (especially if you don't play FPS and don't need the directional queues) - then I'd suggest forgetting the STX and get a reasonable quality external amp/dac. You don't have to spend megabucks to do this either.
Disclaimer - my personal experience only.
I started with mobo sound, then went to a Fiio E7, then E9+E7. I was never 100% happy. Eventually I bit the bullet and bought an Audio-gd NFB-12. It has dual wolfson WM8741 dacs, a very powerful class A amplifier (600 ohm headphones no problem), and a low headphone output impedance (so suitable for high or low impedance cans). The equivalent now is the NFB 15.32 (click link). I recommend this unit without hesitation. I still think it is one of the best bargains around considering features for price.
I do game - and after I bought the NFB-12, I also added a cheap X-Fi (just for the DSPs). So my set-up now is optical (from mobo) to NFB-12, and then either use the headphone out, or I have another amp (LD MKIV) feeding off the NFB12 dac line-out. Even though I've added more expensive cans lately - I still have no intention of changing this dac/amp - it's that good IMO. If you want this method - then you could run your speakers off the RCA at the back of the NFB-12.