Well , I myself is a SE user , but this is a NOKIA territory after all. 

Yesterday a friend brought his 6233 to check out the 5700, the similarity in size and design of the 2 is uncanning , except for the twist of course.

Anyways I did a quick comparison on the SE K800 n this 5700 . Using "smack that " as the comparison song, I liked the sound of Nokia Music Headset HS-44 on both. More bass. Walkmans earbuds sound flater, like Nightstalker1993 said,to compensate for Sonys Megabass thing.The sound is fuller n clearer on the Nokia 5700 (with stereo widening on) than it is on the K800. One can argue that K800 is not a Walkman phone. 

I would say Nokia Music Headset HS-44 is probably a good thing to have. Then again I might have crossed the line n SE fanboys/girls would come out full force n say I dont know what I am talking about.

Anyways, its just a phone.No need to be too upset .If u like SE n Nokia, get them both n keep one spare in your locker. I use the K800 to take pictures mainly..