Unsolvable Woofer Pumping (Phono only)

Well you've certainly taken all the right steps to narrow it down, so good on you. With a lot of the most likely culprits eliminated we are down to only a very few good options left. One being, its perfectly normal flutter, you're just not used to seeing it. Most people with entry level tables have been raised on CD and don't know some amount of this is normal. Not being there we can't know if this is it or not.

Moving on.... your table has a felt mat and let me guess, you're not using a record clamp. So records lay kinda sorta flat. But not really. Problem being, anything other than absolutely flat gets amplified by the time it gets to your speakers by about a hundred thousand times. 

The solution is a record clamp. Not weight, clamp. Specifically, you want one that's cupped on the bottom, or has something like an O-ring going around the outside. Anything to create a space between the record label and the center of the clamp. Then you also want a thin washer, just barely thicker than your mat, that fits over the spindle. The idea being the clamp will press the record down forcing the edges into the platter making the record lay absolutely flat.

A good clamp used like this will almost always be a nice improvement, which since it will also work on future better tables makes it a fine investment no matter what. 

Since you say its noteworthy mostly when just treble this implies to me you don't really notice it with bass-heavy music, which in turn implies to me that whatever flutter you're seeing is no more than normal music level or less. Which to me means perfectly normal, you're just not used to seeing it. But whatever. Try a clamp and see.

@three_easy_payments Nope, it is pretty consistent throughout.

@millercarbon Thank you!  I've grown up with both, but my earlier systems weren't as high quality (I started at about 6 years old with some crappy BSR changers).  I can understand that a little bit is normal, but this seems a little excessive to me.  At lower volumes I enjoy the extra musicality the comes from turning on the "loudness" function.  I realize many audiophiles scoff at this, but, whatever. I like it.  Since it turns up lower end a fair amount, the woofers really show this problem then.

I have the acrylic platter, but no weight/clamp...I've been meaning to get one and can see how it would improve my situation.  I'll definitely look into a clamp instead of a weight though! Thanks.

Here is a video of the intro to Hotel California, at a normal listening level, loudness on. https://youtu.be/sZG5fL4HwN8

It’s sounding like this is probably "normal" performance for this table. With an entry-level table that only weighs 12 lbs, you’re not going to get the performance of a topology that costs more and is focused on design elements such as: high mass in the center of the bearing, incorporation of a low friction bearing, or effective damping that help produce low rumble and smooth operation. I don’t think a clamp is going to help. The KAB will work although many purists feel rumble filters like are sonic bandaids. It just depends on what your goals are and budget you’re willing to spend. It doesn’t sound like you want to spend much if the cost of the KAB is daunting to you.

@three_easy_payments I see.  I will more likely go the filter route rather than clamp, at least until I get a better system.  If that needs a clamp then no problem.  I do plan on upgrading my system relatively soon, which is why I'm hesitant to spend on a filter I may end up not needing once I've upgraded.  I have another thread on upgrading if you'd like to chime in on that :)
I can't see myself ever having more than a $10 000 system. My current one has cost me about $4000.