I have now been using an external device for a while now, and I have to be honest, but I am absolutely in agreement that I have been overly concerned and that he external option was by far the best choice.
I have a number of various options, and believe me, none of them are high end or professional, but some have been proven to be absolutely flawless for me.
Right now, I am using a TASCAM US-100. I do have 2 TASCAM U-120 devices but while they are slightly better, for my needs the 100 seems to be the better choice.
I also have a few other similar devices, most of which are MIC & Guitar inputs but also have USB Audio and they have varying levels of good to bad audio.
IO also have the older Firewire ones that I used to love, and while I was using a basic amp, they did nicely enough, but once I got a half decent amp, I realised they are absolutely awful.
Now, back to the Tascam, I dont know if this is capable of playing anything at any specific frequency, and I dont know if it switches in and out of any specific frequency, but it just plays what I ask it to and I have not had a single issue with it.
One huge bonus, is that the Creative card is stil in the PC, and if I unplug the tascam, the creative card continues playing almost as if its unaffected and the same when I plug it back in... They switch almost instantly between each other, and that I like.
I will admit however, that I have not used the Tascam on the AMP much at all, its mostly through the headphones.