The STX550 is a 3.0L V6 with a 6-speed manual or a 7-speed auto.
I wonder if they have SCV issues like the 4-bangers. I wouldn't suspect that at first, simply because of what you've just done - while faults can occur at the same time as other work and cause confusion (eg a tail light blows after you've plugged in a brake controller) the general approach to problem solving these is to look around the area you've been working on.
Fuel filters have been a little problematic with the seals - if you don't get them right, they can leak. Another issue could be how tight the hose clamps are on the hoses around these devices. If you over-tighten the clamp, you can cause the hose to pinch, which will allow air to be drawn in on the suction side (naturally the pressure side will be obvious with the amount of fuel pressure these babies develop!).
+1 on looking back over the work you've done, it doesn't take much to cause a leak. Our fuel systems run at extraordinary pressures - which also means they draw fuel from the tank at a very high rate. This means that if there's an opportunity for an air leak, it will happen very easily.