Cart update: old TT, new electronics

Sdecker, congratulations on your uprade.
The conditions you describe as well as the sound definitely indicate to me that you are suffering from not enough mass with your tonearm. I tried to find a picture of your arm but was unsuccessful, but perhaps I can share some tweaks with you that I have used in the past, but first, I'll try to explain your objective(s).
The complience is of corse, the stiffness of the suspension of the cartridge. You can substitute 'weight' for mass for concerns of the arm. The 'weight', if you will, affects the inertia of the arm, as it resist movement.
Every combination of complience and mass produces a resonent frequency that is the natural resonence of that combination. If it falls in the audioband, every time that frequency is produced it will vibrate in sympathy. The result in the range here is masking of those particular frequencies. You want to make the arm heavy enough to drop that frequency out of the audioband.
You can do it by adding weight right at the cartridge. As you move back toward the pivot, you will have to add more weight to equal the inertia of weight added at the headshell. Also, the more weight you add at the headshell, the more weight you need for counterweight.
electrical tape is a good substitute for armwrap. You can wrap it around like some commercailly avaivable do, or you can run strips straight with the tube. If you put it on without tension, or too much tension, it will stay put. This will help damp the arm from ringing.
You could also stuff the armtube. I have used a drinking straw and cotton on both the audioquest and immedia with good results. The straw is to isolate the wire from being smashed against the armtube and/or stuffing, which could be an undesireable dialectric for the wire. You could do variations, I have found cotton to work the best. Many tonearms just use packing foam, you could try that without a straw.
If your arm has a separate tailpiece (the part the counterweight slides on) it may be hollow. If it is hollow, you could stuff that.
If you can get a hold of some lead sheathing, with is available from some commercail construction suppliers, that could be used for a variety of things. it can be cut to any size, and is less than 1/16" thick, and quite heavy and flexable. I have the counterweight of my rega wrapped with it, so it is half lead. It is just taped tightly on, and it works well.
If you go to the hardware store, you could find some collers that have an allen screw in them, sort of like the fancy counterwieghts for rega's, just not as heavy, but this would be a cheap, fast way of increasing the couterwieght. Find them at ace.
I hope at least some of these help. It sounds like you already got into the thread of twl's tonearm tweak, "strange tonearm treak long". If you get your resonence down closer to where you want it, you could try some variations on some of the different explanations of why it works so well. If you were to to some similar or separete variations of twl's tweak on your arm, you could not only possibly improve your arm but help in explaining or proving the theories behind it.

Arm mass confirmed.

(A line got cut from my previous post: #1 was reduced audio bass output vs my old cart, #2 was increased infrasonics)

Thanks to basement's suggestions. I didn't have quite the materials on-hand last night for a more elegant solution (next week), so I cut a pencil to the length of my tonearm from headshell to near-pivot (4"?), added a screw for more ballast at the front, measured it out to be about 6g, and electrical taped it atop the tonearm. This raises my arm mass from 7.5g to around 14g, and it's along the length of the arm vs concentrated in one place. The arm itself is sealed and way too narrow to put anything inside of it (think black widow).

The good news is the low-end is transformed. From the lower mids down, a newfound output, weight, detail, focus, image stability. And below say 100Hz, all the way to the lowest fundamentals I could find on my bass-heavy records, it was like adding a subwoofer. Very satisfying and confirmation of the whole compliance/mass matching. It also subjectively changes the whole balance of the cart, now sounding less bright and lightweight as the lower half is now in balance.

The bad news is it didn't improve the huge infrasonics I'm getting from the setup, perhaps made it a bit worse. The massier tonearm limits the cart's lateral motion so more in-phase audio bass is reproduced electrically rather than mechanical movement. But for out-of-phase vertical LP ripples, seems the added mass has the same effect, forcing sub-audioband bass into electrical output rather than tonearm movement. I guess it must be Grado's mechanical frequency response extending to near-DC? I don't know why all phono preamps don't have at least a token sub-20Hz filter...

Okay, what you need to do is increase the lateral inertia by taping the weight onto the bearing housing, and not the arm. This will increase the lateral mass enough to retain the sonic improvements you heard, but will not affect the vertical mass, so warp tracking will not be adversely affected. This is the basis of the "Strange Tonearm Tweak" thread that is on this forum page. The weight must be increased on both sides of the bearing housing equally, so as not to affect anti-skating. I will be marketing a version of this for the Rega Tonearms shortly. It has been tested and approved by several Audiogon members. It will work on your arm too, but you'll have to figure out on your own how to make it. Mine only fits on Rega arms.