Ilv, I have been using either the Pass Labs X-350.5, and recently a pair of XA-100's in my system and both have sounded gorgeous with the Placette Dual Mono Active Line Stage.
I'm a great fan of Nelson Pass designs, his preamps have alot to offer, however, in my system, the Placette allowed his power amps to really shine in my system. Guy Hammel's, that Mr. Placette, other preamp which is a passive line stage offers about 80% of the performance of the active linestage and costs $1500.00 compared to $4500.00 for the active linestage. I auditioned seven different preamps, both tube/ss, and choose the Placette because of its sonic virtues and overall performance.
I'm assuming you love your new X-250.5, Pass really nailed it in the X-.5 series, they really are beautiful sounding amps. Well, I hope you put the Placette on your audition list, you get a 30 day home trial period and Guy is a great gentleman to do business with.