I run my tanks in parallel but I have a friend that runs his tank in series; there have been several posts that have argued this point and I do not wish to add to them. I will try to lay out the facts as I know them and you will have to make your own choice. I use a parallel setup because I believe it is important to keep the airplane as balanced possible for best flight performance. Most guys that fly with them in series say they do not notice any difference in the airplane throughout their flight. I'm guessing that this would be dependent on were your tanks are located, I believe that as careful as we are to ensure that our planes are balances on the CG how can you say that having about three pounds on one side of the aircraft doesn't affect the handling in some way. You can make your choice. If the fuel tubing in your tank is to stiff now it will not get any better by soaking in fuel. Normally fuel tubing gets stiffer with age. Try a different brand of tubing BVM sells some good stuff. Give that a try.

PS: with a parallel setup it is important to keep everything symmetrical, fuel and vent line lengths, tubing diameters. Then test it for even feed and adjust line length if not even; more work but I think worth it in the end.