I haven't had the K 140, but the vintage K 141 is a really great pair of phones. I rarely listen to them, but they always sound so mellow and nonsibilant – yet never boring. I dig the slight depth to the soundstage as well. They're not the greatest thing ever, but really neat for what they are, and I don't think I'll ever sell them. I also finally gave them a pair of hair elastics, so getting a good fit again.
Here's some graphs of the K 340, with 1/3 octave smoothing:

Red line is for the K 340 with pleather pads and no cotton in the cups. Blue line is with pleather pads and some amount of cotton in the cups (maybe a third of what was originally there on my pair). Green line is with velour pads and the same amount of cotton as for the blue line. Measurements were taken with a home-made artificial ear, and each line averaged from three different ear positions.
The red line was measured about a week ago, and may not be totally comparable with the other two, which I measured today. But it should give an estimation.
I think my pair may be bass-heavy, or in any case probably not bass-light. The way I got them, with pleather pads and a whole lot of cotton in the cups, they sounded really bassy and quite dark. I took out all the cotton, which helped make the phones less dark, but they still sounded veiled in the highs. I've had a lot of trouble finding suitable cans for my pair of AKG velour pads, considering how much they emphasize the treble, but I think the 340 I have is pretty good with them. Pleather pads seem to deaden the sound compared to velours on this 340, and I suppose the energy difference around 10 kHz is what does it.
Anyways, try velour pads if you find your 340 too veiled. I personally think the velours fit the look of the 340 really well, and I like the sound, too.
Edited by vid - 2/18/12 at 3:45pm