From all the diff. reviews i've read online I can say that K750i beats K790i in loudspeaker volume when playing mp3s not ringtones. But this is the case with walkman series also. Since, w810i I don't know why they have limited the dB level of the cellphones.
And as far as sound quality goes Digit reviewed it to be the *best* music phone available *when* paired with a set of good earphones instead of the bundled ones.