" A 6 wt single-handed line weighs 160 grains, give or take 5% or 8 grains. A 6wt Spey line can range from 400-600 grains depending on length of rod."
I use a 425 grain on my 6wt switch. That line would probably snap my single hand 6wt at the cork.
Edited for clarity. From Gink and Gasoline:
"Switch rod and Spey rod weights are rated on the AFTTA standard and single-hand rods and lines are rated on the AFTMA standard. It’s like comparing Celsius to Fahrenheit or meters to feet. This means that a 5-weight switch rod, for example, takes a much higher grain weight line than a 5-weight single hander in order to get the proper load. That 5-weight single-hand rod needs a line in the range of 180 to 200 grains, whereas a #5 Switch rod needs 300 to 330. That’s a huge difference and if it seems unnecessarily complicated, that’s because it is. It’s critical when choosing a line for your switch rod that you think in terms of the grain weight needed to load the rod."
I am new to the world of Spey and Switch as well. I just spent a week up on the Rogue River in southern Oregon 2 handed casting for steelhead. A bad line put on an amazing rod will make your rod feel like absolute dookie. A good line matched with a mediocre rod is incredible. A great rod and line pairing is life changing.