I agree, Never. Your description certainly "sounds" generally like "floor coupling" to me, too. Your R500c 2x10" weighs just 36.5 lb.s, so it is not quite as firmly "affixed" to the floor as your 46 lb. R60 1x12". And, although amplifying the same root frequencies, the two amps' differing weights, speaker sizes, cabinetry and circuitry will produce somewhat differing overtones and harmonics which can behave quite differently.
Is this the only location you've noticed this? What is the floor like? Wooden slats on wooden joists suspended above an earthen ground, or perhaps even above another room below? Such flooring would provide some likely resonant spaces to reinforce certain frequencies, or harmonics thereof. Or, if it's some thin covering over a concrete slab foundation, then the acoustics may moreso involve the walls' proximity and construction type.
I would think the same acoustic processes are operating on the R60 as on the R500c, but perhaps the R60's overtones and/or harmonics are just not as prevalent or noticable to the ear. At any rate, I can only suggest elevating the R500c in that position/location in the future.
Granted, I'm not super knowledgeable about such things, Never. But I have occasionally dealt with such things on, at least, a superficial, or temporary, basis over several decades at different venues. And you have my 2¢ worth of explanation, sir, if no definitive recommendation. I hope it helps.
