I currently use a jl 300/2 amp to power my focal utopia's. But i was wondering if instead i used a phoenix gold xenon 400/2 amp would be an upgrade and provide higher spl and better sq?
the JL is by far a superior amp for design and quality. as for SPL, the 300/2 is underrated so it probably in reality puts out at least the same power as teh PG amp.
regarding SPL, you need to double the power to gain as little as +3dB in SPL. to audibly double the volume (+10dB) you need to increase power by ten times. the point is, going from 150 to 200 watts per channel would hardly be noticed.
If you want better sound quality consider gerring a second 300/2 and biamping the focals. If your head unit has the adjustability or don't mind adding something like the audio control dqx then that would be the way to go.