Headphone volume: Can it be increased by an app?

For a change I'll state my question first and leave the potentially boring details until last:
Is there any software that would improve the sound volume from my computer?

The boring details:
I'm running W7 Ultimate 64 bit on an Asus board (specs below left) with inbuilt Realtek audio.
I use fairly basic Sennheiser HD410SL headphones much of the time. These are connected to an old Altec Lansing computer amp that has a woofer and two desktop speakers. Most of the time, the sound from the headphones has a reasonable volume, but some material - Bill Maher's website for one - I strain to hear, even with the volume switch on the desktop speakers at full volume. Incidentally, just in case someone asks, I've just bypassed the Altec Lansing hardware and connected my headphone lead direct to the motherboard. The sound level is worse.

So I wondered: Are there are any apps that would amplify the output from the computer?

Hi. That's a good question. The Altec Lansing sound system is plugged into the computer's audio-out (green) socket.
The cans are plugged into the headphone socket of the Altec Lansing right-hand desktop speaker\control, which also has buttons for ON-\OFF, tone etc.
But I've just realized that the computer tower has a headphone port on the front of the box, which I've never, ever used. I'll try plugging in the cans there, which should, I guess, then utilize the Windows volume control. I'll come back after that. Thanks for the suggestion.

Go to Control Panel > Sounds. Double-click the sound device, click Enhancements and check Loudness Equalisation.

townsbg: Thanks, you were right. The Sennheiser cans have high impedance. An old Sony set solved the problem.
SAVVYSCLUTTER: Thanks. Changing cans fixed the problem, but I'll try your suggestion and see if can get more boost.