K240 - comfortable for long periods, lacking some sub-bass 'fun'
I would chose the Superlux HD681 over the K240 for sheer price to sound performance ratio and similar build quality.
Sennheiser HD4.20S sounds VERY similar to the HD451 (both are good) but with a slight boost on bass and treble.
The HD4.30 is a good alternative if you love bass, as these are very bass oriented.
Hope this helps a bit.
No idea about the other two.