Marantz TT-15S1 anyone with experience fitting an electronic speed box?

I think that the first thing you have to know is whether your table is currently running at the correct speed.  You need to check with a KAB strobe.  If it is, I think a speed box will bring relatively little. Most listeners aren't that sensitive to relative pitch, unless it's way off.  However, people are more sensitive to wavering pitch, especially if you listen to a lot of solo piano and stuff with sustained chords.  IMO, a speed box won't help if you have off-center records or a bad belt or whatever.  It does make the motor turn at the correct speed, but it can't control the platter speed if the mechanics of the table are off.  But check your table with the strobe and see where you are before doing anything.  

Also, I have the Phoenix, which offers a much finer speed adjustment than the Speed Box.  it's not just an intellectual exercise - I found I could not get the speed perfect using the coarse .1 increment found on the others and needed to go to the finer adjustment to get it dead on.  
I like the Phoenix a lot, BTW.  But then again, my TT started running very fast for some reason, so I needed an outboard box of some sort. 

Ok - then I suggest you buy one and listen for yourself.