Hi Chik,
Prior to spending any money on different op-amps for your PB2, I vigorously recommend that you get yourself a good USB DAC. You will be amazed at how much better everything sounds when you bypass the sound card. Using player software such as Foobar 2000 in combination with the WASAPI plugin can also bypass the operating system's handling of your sound - for a more direct path to the USB DAC.
You are currently double-amping - simply amplifying the sound quality (or lack of sound quality) produced by your sound card. Giving yourself more headroom by increasing the PB2's current or voltage will not fix what's wrong with your current setup.
I know that the cost of a few op-amps might be more appealing than saving for a USB DAC, but again, in my opinion, a DAC should be first on your shopping list.
The
Stoner Acoustics UD100 costs about $60.00 with shipping. It's got the same ESS9023 DAC chip found in the $280 Audioquest Dragonfly, with some major differences - the UD100 DAC does not include an analog volume control, but that's OK, since you would be using your PB2, rather than plugging headphones directly into the DAC, and the UD100's USB receiver chip can only handle redbook audio (44.1 kHz / 16-bit), where the Dragonfly's USB receiver chip can handle hi-rez files (i.e. 96/24).
If you don't have any hi-rez files, the UD100 is a pretty good deal as your first USB DAC.
An equally affordable ESS9023 DAC that actually offers 96/24 capability is the
HiFimeDIY Sabre USB DAC.
These 9023 DACs will sound a little thin and lack warmth compared to some more expensive DACs (like the DACport LX), but they deliver plenty of detail, and you can "warm" them up with your PB2, by experimenting with op-amps, later.
I hope I haven't been too forward in telling you how to spend your money, but feel free to solicit other opinions and enjoy the journey!