I have read recently that the simpler the signal chain the better the reproduction of the source sound. I have a decent sound card whose op amps have been modded. Am I gaining anything by going through a tube preamp? Should I just come out of my sound card and go into my amplifier? I realize that using a pre allows for ease of volume control and most likely better impedance matching but are those the only advantages?

Here is my current two channel setup

Asus STX sound card (op amp upgrade)
YS Audio Experience Symphonies Plus Preamp
NAD 2400 THX Amplifier
Allison Four large bookshelf speakers

Thanks for your time and expertise,

Jason