K619s arrived today. Haven't been able to burn them in yet so this is a bit preliminary.   Overall in terms of sound quality, they're not worlds apart. For me the 619s have a wider-sounding stereo field than the K451s, better clarity in the upper range, however they're quieter in the low...

Sorry to bump a kinda old thread. I've just ordered the K619s and my last phones were the K451s. Will try to come up with a verdict when they arrive.