Using the same headphones, i too, prefer the sound of my X-Can V2 to that of my vintage gear, but even i use my X-Can to drive my SR80i, it still doesn't as good as my PS1000 through the vintage gear.
I know that it's unfair to compare the SR80i to the PS1000, but as i have said, if i had $600 to spend, i'd rather get some PS500 and drive them of my old stuff, than buy some SR225i or SR325is and spend the rest on an headphone amp.
I think that they're is more sonic gain to be had by upgrading the headphones, that being said, if you own TOTL headphones, or, let's say that after hearing the pricier models, you still prefer the sound signature of your $300-$400 headphones, then buying an headphone amp makes sense.
Just my two cents.
I use an Asus Essence STX souncard to drive my RS1i....they sound great.
I have listened to them thru my buddy's Schlitt Magi/Modi and I have to say the Asus sound card is every bit as good IMHO.
I have thought about buying an amp to set up a listening station in the living room using a laptop.....not sure what amp would be good...I would not want to spend over $400 on one, for sure.....as I would love to add the Grado PS500 soon.