Excellent points by the others.
I'm guessing my volume in low gain on the ARC is due to running BAL (51dB) compared to BAL on the Rowland Cadence at 64 dB. On to something there?
Yes. As ActusReus indicated, 51 db of phono stage gain is too little for use with a 0.3 mv cartridge, at least in most systems.
I see that your Synergy IIi provides adjustability of its overall gain for each of its inputs over a very wide +/- 20 db range, in 0.5 db increments. Have you experimented with different settings? If not, that is where I would start. Perhaps reducing it for the high gain setting of the phono stage, and increasing it for the low gain setting, and comparing both sonics and noise levels for various settings.
I would NOT suggest that you change the preamp's input impedance setting to 600 ohms. That is much too low a value for the phono stage to drive properly.
Regarding possible upgrade to a Ref 5se line stage, I have no particular knowledge of that preamp but my guess is that it would not help. Its maximum gain is 12 db balanced/6 db single-ended, which I doubt is enough for use with the low gain setting of your phono stage; its noise performance may not be any better or even as good as that of the Synergy; and the limiting factor with respect to system noise performance when a phono source is being used will generally tend to be noise that is generated or picked up where signal levels are lowest, i.e., in the phono stage and/or the phono cable.