The gfp750 mod does only apply to the active circuit.
I'm using an ARC LS 16 with my Marsh A400s through some VMPS speakers and love it. Very smooth and transparent sound. The Audio Research does not lose any dynamics or extension at the frequency extremes. If you are looking for tube euphonics you'll need to try something else as the ARC has just a taste of it. I tried a Cary and it had a lush midrange but dynamics and detail were lost. The ARC loses no detail and is Very smooth. Wished I would have tried a tube preamp years ago.