The GFP-710 (along with every other standard Adcom preamp) uses op amps for tone controls (bass/treble) as well as the audio output stages. I used to have a GFP-565 a long time ago. It was okay and nice, but nothing special.
The GFP-750 is a completely different beast. It was designed by Nelson Pass and uses all fully discrete analog circuits (no bass/treble tone controls). I would suspect that it's significantly better than Freya with stock tubes, but we are also talking about colored/textured tube sonics vs. solid state. Now Freya with upgraded tubes could be a different story (such as PSVANE or some true NOS Amperex, depending on your desired taste).
The GFP-750 is a completely different beast. It was designed by Nelson Pass and uses all fully discrete analog circuits (no bass/treble tone controls). I would suspect that it's significantly better than Freya with stock tubes, but we are also talking about colored/textured tube sonics vs. solid state. Now Freya with upgraded tubes could be a different story (such as PSVANE or some true NOS Amperex, depending on your desired taste).