I've owned my 1266TC for about a year and it's one of the best headphones that you can buy. The TC needs a good amount of power to sound its best. I've talked to Joe Skubinski, ower of JPS Labs, and he told me that the headphones need about 2 W/Ch or more. You won't hear what the 1266TC is capable of delivering out of the Luxman integrated from its headphone jack.
One thing that you could try is to get a headphone extension cable with connections to your speaker binding posts. Since the Luxman has speaker binding posts for 2 sets of speakers, you can connect your speakers and headphones to the speaker outputs. Doing this should give you more power than using the headphone jack on the front of the amp. To do this you'll need to make or buy a headphone extension cable to connect to amp speaker binding posts with either spades or bananas. On the headphone end of the cable, you'll want either a 4 pin female XLR connector or a female 1/4" (6.3mm) jack. Check what cable you'll get with the TC. My TC came stock with a 4 pin male XLR and a 1/4" adapter. If you try this, be careful when switching from speakers to headphones on your speaker selector switch. Always have the volume control set to the minimum when switching from speaker to headphones or vice versa.