I do what you are looking for by running the LFE out from my processor to the L RCA input on my Velodyne HGS-12 and a second pre out (My pre has dual outputs which is convenient for this configuration) to the R RCA input of the Sub. I then Set the crossover on the Sub to around 70 or so. I set everything up optimal for two channel and then adjusted the processors LFE output level to match that. I'm know I am loosing a little of the LFE signal by applying the crossover to that signal but the loss is worth it in my opinion to get the sub to work for 2-channel as well as HT.
Hope this helps.
Hope this helps.