I own neither, but here are some basics, hope it helps with your decision?
the 701 is obviously much older with only 5 hdcp 2.2 inputs, not sure how much longer they will provide firmware upgrades, it does have a little more power, class D amps but has the superior mcacc pro room correction solution.
The lx503 got great reviews for power and phono stage, I imagine the lx504 is probably just as good. I believe it has the new Onkyo hdmi board, has much better dacs but sadly they only updated it to mcacc advanced, which has a universal crossover. At least the standing wave control is supposed to be pretty decent.
I like the lx504 for features, being future proof (for now) and the fact that the lx503 had such great reviews.
Last edited by eriksells916; 09-18-2019 at 08:29 AM.