I own a Lamm LP2 Deluxe, but haven't heard the ARC (though I far preferred the Lamm to prior ARC units I had heard, just seemed more like music, the ARCs being more to the analytical side). To my aging ears the Lamm is slightly dark-sounding, excels at layering of images in a soundstage, is tonally correct, has excellent dynamics and, as Damaradona points out, is very quiet. However, part of that quiet is because it only has 57db of gain. Kurt tank is correct, it can be magical with some cartridges, but its limited gain means that it may disappoint with very low-output MCs (I ultimately gave up with my Koetsu RSP, just not enough volume to convey power music or records cut at a low level). And the inability to play with the impedance is also a drawback. I solved those problems to my satisfaction by getting an EMT cartridge with its matched step-up transformer, though there are certainly many cartridges that will work fine with the Lamm by themselves.
Hopefully some PH-7 owners will give their observations for you.
Hopefully some PH-7 owners will give their observations for you.