Memory Foam Topper on Purple?

I feel like an idiot on a few levels, but I'm also an idiot in constant pain so I'd love a solution...

tl:dr; just need to know if using a memory foam topper on a Purple mattress screws up some magical inner workings for this sucker.

I bought the original Purple mattress last year and tried it out for a while; I sunk in a little, but not that much (which surprised me; I didn't exactly want to be swallowed by a mattress of course, but I had a purple pillow/seat cushions, and it just wasn't the same). So I just thought "ah, maybe I need to upgrade to the Purple 2, that's my problem. It's not providing support beneath or something, I guess." I traded in the original and got the Purple 2.

I'm primarily a side sleeper, and I'm 150 lbs. The first few nights on my Purple 2, it felt great! No pain at all, everything's dandy. But I noticed that I still didn't really "sink in" at all - I felt like I was on almost floating top of it, even with the official mattress cover and sheets (which I also grew to hate: I roast in those sheets. I miss crisp cotton sheets :( ).

Then, it started to hurt - a little at first, and more and more over time. My hip would just feel terrible pressure all night. I can only sleep on one side for a few hours, then I have to roll over to the other side. My knees hurt a ton and I have to sleep with a pillow between them now, too. Even sleeping on my back doesn't really work; I don't like to sleep that way for very long, and I start to feel strangely numb after a while on this mattress.

I really, really don't want to deal with another trade-in deal with Purple (I'm outside my 100-night window timeframe anyway); it was a huge hassle to do the first swap out (not because of their business practices, to be clear, as they were perfectly pleasant; it's just the practicality of lugging mattresses up and down several flights of stairs, dealing with pickup, waiting on fedex to deliver whenever they get around to it, etc). I also don't want to have to just trash this and buy another mattress right now, as that's $1600 down the drain.

Can I at least throw a memory foam topper on this or something? Has anyone done that? I honestly thought I was treating myself with a new mattress when I "upgraded" to Purple, and now I'm in hell. :(