Things might be a little quiet on the forum because people may not sure what you're trying to do. Your AV receiver is an amp designed to drive your speakers, so if you wanted to get the best quality out of your headphones, you'd normally want to leave the AV receiver out of the loop completely and connect your sources directly to the headphone amp.
What sources would you mainly be using your cans with? The number of sources and the type of outputs they have will probably determine/limit what headphone amp you go for.
You might consider these:
Project Head Box DS £300
Denon DA300USB £329
Cambridge Audio DAC magic plus £300
Alternatively, does your receiver have a digital output?
Edit:
PS - do be careful if you want to "boost the volume" of your headphones. Particularly for long listening sessions, it's quite easy to damage your hearing. Better to go for an improvement in quality alone as your LX55 should already have plenty of volume through the headphone output (unless there's something actually faulty with the headphone amp). You want to be able to enjoy listening to music for a very long time - could be tricky if you lose your hearing!