I've spent my share of time with this little gem as well, and have come out with two lines that really shine so far and am anxious to try more as spring arrives. Unlike the rest of the Expert series or Customs and Specialists, this rod is quite gentle where the rest have a higher ratio of guts in reserve. I particularly like to use it to clean up my technique after throwing heavy rods and lines; think of the football player who does ballet to keep an element of grace in his game. :hehe:
Simon's 5/6 Windcutter is spot on for me on this rod, it loads the rod well yet lets the rod have a little authority over it and they get along real well.
Hardy's 8/9 Mach I Salmon line, even though it's rated 8/9, is a beautiful match for this rod as well. It brings out a very delicate balance to the rod's action.
coho48 - I would guess that the XLT 6/7 is significantly over-loaded for this rod, but will have to check the grains / head profile to venture more than a guess. You can see that the line ratings in spey casting are deceptive at best, instead look for grains over length of head to match with the rod.
The Windcutter 5/6 is only about 350 grains over 54 feet, and it works well for me on this rod.
You can also cast the 6/7 Midspey on this rod as Brian says which has 490 grains over 65 feet, but it will require that the lift and sweep to form the d-loop is super smooth, the rise at the end of the stop is adequate to keep a light anchor, and the shape of the d-loop fully extended on both halves, energized. Otherwise the weight and momentum of the line, if not well formed and directed into the same plane, can overcome this gentle trout rod.
The Custom 7/8 feels a touch beefier in the hands but has a deep reserve of power and is a true steelhead rod.