Headphone amp

I have the Fostex TH900 headphones and I am using an Antique Sound
Lab combination Pre-Amp and Headphone Amp. Will getting
a Woo WA22 headphone amp greatly improve the sound ?
Yes, a dedicated headphone amp helps.

The WA22 only drives headphones up to 600 ohms though. Have
you considered a more powerful unit? (considering you buy
some planar headphones in the future) You can find the
Burson conductor for less, and it have 4 watts of power.