I recently bought Richards' Valenti J4, lovely Bass.  Now that I've had a chance to play around with it I'm considering changing the pre amp.  It has an OB-1 with a pull pot for passive mode but no control over passive tone.  I'd really like to be able to roll off in passive mode as I like this sound.  So my question is, does anyone have any suggestions/experience with pre amps that have this sort of control?  
 
I've had a look at the East J Retro Deluxe and it does seem quite appealing.