[alstone]

The only thing "fun" about the 23's is taking them off after you've listened them. These sound horrible. Awful. Do yourself a favor and skip these.
 
Having said that, at least the headband is long enough. 

 

[povidlo]

These are legit, another hidden Koss treasure. Stock pads are OK, I replaced with Brainwavz pleather. Homey, warm, thick, bass-focused sound. Too thick for tubes and R2R, but just right with a regular delta, solid state source.

 

[Sunstealer]

Many thanks to @AnAnalogSpirit for the tips on modding. OOB - muddy, closed-in, restrained.

Took off the rear clear sticky disc - immediate improvement in the mids and treble but still muffled. Cut a disc out of the front pad foam - treble restored, really clear, no sibilance or harshness. Still quite bass boomy though.

I then tried a pair of THX00 pads for fun. They fit loosely around the cups (not in the slot). Boominess, spaciousness and cohesion improve a little, I think due to the angled nature of the pads.

I then padded the original earpads in between the foam and the pad itself to create a slight angle and more space - as stock the foams sit on my ears and it is uncomfortable after a while.

Much better but still too much midbass and boom for extended listening - enjoyable for a few tracks though.

I though about damping the cup with the circle of foam I cut out but this would occlude the rear port of the driver. On closer examination there are 8 tiny rear ports in the cup, making this semi-open as has been stated.

I taped over 6 of these ( the 12 o'clock one is occluded anyway) and this has kept a healthy dollop of midbass but not so much that my head vibrates.

Perfect.