I got it really really bad on my old monty x-lite, hurt for absolutely ages afterwards for months... couldnt get rid of the ache, even when i hadnt been riding for a while. Sold it because of the ache, got a python...got a let better. ended up selling that aswell, got a stock, no back problems at all (Y) . I put it down to having a short, low bike.
Was basically like mini mans when i had my monty - hurt absolutely loads in the morning after riding.