The fans on Flex-ATX PSU's come in 2 varieties; 40mm fans on the back, or 80mm fans on the bottom. (Just like micro-ATX's) You may want to shop around for one that has an 80mm on the bottom and then switch the fan with a queiter one.
That may be a relatively simple way of making it quieter.
The PSUs i mentioned use 80mm fans. Part of the problem with the Fortron was that there wasn't space for a 25 mm thick fan, but that was easily solved by mounting the fan on the outside. The real problem was the coil buzzing. Don't know if there is enough space in the Seasonic for a 25 mm thick fan as I haven't seen any reason to open it yet (same model MikeC use in his breadbox). The Seasonic has a slight vibration when it speeds up, so I might need to put something between the case and the PSU, but I need to experiment some more. The PSU is mounted about 10 cm inside the case (next to the CPU) with walls to the backside to prevent the air from returning into the case, but there is some work left on this.