The tell was that it was coming out of the outlet side of the Jandy like a fire hose, and, at the same time, filling the slight window on the Hayward when doing so.
It was an obstruction in the outlet side @ the pool. Funny enough, there is nothing in the mega shop-vac... so I'm not certain what that obstruction is or was... and I don't think I care. LOL. There are no trees, etc. in the area... so... Yay.
Thanks again for the help... even the "what's a Jandy or a Hayward" comments. They made me laugh.
What no one has addressed is that the SP2300VSP is a medium-head pump. They are great for moving a lot of water when there is little restriction but the flow drops off much faster than with a high-head pump when there is. A dirty or worn-out filter cartridge can cause the issue that is being experienced. A 100 sq. ft. filter is considered very small by today's standards. Replacing the cartridge would be a great start if it is older than 3 years. A new, larger filter would be better.