Sorry guys.  I didn't look at the outputs on my Adcom amps or Adcom pre-amp before I posted.  Looks like it won't work.  My intent was to some how use one of my Adcom GFA-555 amps (I have 2 of them)  to power the headphones using the headphone-out jack on my Adcom pre-amp.  I dug the Adcoms out of the closet and now see that there is no way to send the high output from the amps back into the pre-amp and through the headphone jack.
goobaah - Since I can't use my GFA-555's, I'll ship them both to you.  Just provide me with your address...along with a $1000 money order to cover shipping.  I'll pack 'em up real nice! 
 
 
FYI - I got the Sennheiser 650's in the mail today.  They sound 'good' plugged into the X-Fi Titanium soundcard in my PC, but definitely under-powered.  Looks like I have to pry my wallet open and buy a headphone amp.
  
Any recommendations on a headphone amp from someone who currently has a pair of HD650's?  
Also, why do I need a DAC?
Here are the specs of my PC - 
Mobo - ASUS Rampage II Extreme
CPU - Intel i7 965 Extreme
RAM - 6 GB Corsair Dominator PC3-1600MHz
GPU's - (2) EVGA GTX 285's
HD's - (4) 640GB Western Digital Caviar Black's
Audio - Sound Blaster X-Fi Titanium
OS - Win 7 - 64 Bit
PSU - Corsair HX1000
Monitor 1 - Dell 30" 3007WFP
Monitors 2 & 3 - Dual Samsung P2370 23" displays