rocray,
The First Watt F5 definitely has more "meat on the bones" than the GaN 400. But if you start doing an A/B comparison with other First Watt amps I think you will conclude that the F5 isn't a particularly nuanced or accurate amp. I felt it had a very specific sound that was a lot of fun to listen to but ultimately I replaced it with the Aleph J. I think First Watt amps in general have very good soundstages and imaging but the GaN 400 truly excels there. Comparing it to the M2x, I often felt like the sound was being unveiled. It is the most appealing part of the GaN 400. If you have heard the F6 I would say the GaN 400 sounds most like the F6 with a kind of purity and clarity that is fairly unique. But it does come at the cost of that fat, liquid, rounded sound that good tube amps give you. So far I have yet to find the perfect amp but then I'm not willing to spend 5 figures on an amp either so it may well be out there. And remember, a lot of this is personal preference. If you want that bass slam that the F5 offers then that would be a better fit for you.