sword, you have a very similar system to my own. I am wondering, since you said you ran the optical setup in audio creation mode for asio, I am assuming the card is capable of bit-matched playback...If so, using the H701 optical input there should be a very healthy output to your amps, depending on where you have the attenuation on your crossovers in the 701, and how your gains are set up...this should be more a matter of matching gains between the 701 and your amps than it is a question of the worthiness of the card when it's in this mode. I really don't understand how there could be a volume difference on the Toslink output unless and until you use the volume controls in software...My suggestion would be to use Toslink in bit-matched mode, leave the volume controls in software alone at 100%, then use the 701's volume control to adjust as necessary. If you have your 701 and your amps set up for the analog output of another card, then there will almost certainly have to be some adjustment to your amp's gains necessary, but I've never had a problem getting clean output of the 701 to a level that I can't match with the gain setting on my amps (JL slash series).
Also, given that the 701 is in the signal chain, I don't see the point in fretting about analog out from the pc, unless you're planning on circumventing the 701...you have a bit-matched digital out available from the PC, and and sound quality of DACs and such would ultimately depend on the processing capabilities of the 701.
In any case, perhaps I'm missing something...is 3V really too little for you to find a gain setting that will push your amps, when most amp gains run, what, .2 volts on up? Regardless of the advertising, there are only a few head units out there that really do much better than that...and it's not just about voltage out, it's about how clean you can make that voltage. There are a number of Pioneer head units, for example, that run "low" voltage pre-outs, but offer a clean sound that's easy to match with a little gain adjustment at the amps. It's all about control of noise in the signal chain, and max voltage out is just one measurement among many that should be considered.
189? Is that a typo? I've seen people on 5k watts not hit that.
http://www.soundoff.org/result.htm