Doing the ab between a sony 7506 and the superlux - the biggest difference is that the sony is much more recessed and subdued than the 661 which is more forward and aggressive - I actually think its a tad brighter and sparklier at the top end than the sony and more slam than the sony, perhaps a bit more v shaped. I would say sony has a slightly more refined reproduction but lacks the impact of the 661 which is more musical in that respect.
So far just tested them out with dance music and will move to some more challenging vocal tracks to see how they both perform.
Running them out of the superlux ha-3d amp which has three jacks

Ensures I get a totally equal setup.
Comfort factor sony wins hands down but this is the most refined superlux product to date for sure.
Sadly the 661 does not fold but the cups swivel and go flat.