Did the mod to the SL1200MkII involve changing the tonearm?
While that table is a classic and a lot of fun, subtlety is not among its attributes. Saying that you are not terribly impressed with a cartridge's performance on Technics SL1200MkII is roughly akin to complaining about your new tires' handling at high speed on your load laden Mack dump truck. A Mack dump truck is perhaps the finest machine for hauling earth from place to place, but it is not designed for performance. If you want performance, get a BMW, Corvette, or Lexus or even a Honda Civic EX. (Please don't carry dirt in them).
The Technics, like the Mack truck, is an industrial product, built for the rigors of heavy duty use. It was made for DJs and radio stations; designed for 24/7 use. It comes up to speed nearly instantly and has a very accurate speed control that can be used to match beats or shorten top-40 songs. It can take abuse. It can be back-cued. It can be transported easily. It weighs a ton. It's arm is built to take abuse and handle cartridges that are massive and unresponsive. It does not skip, ever....but it also does not sound good under any circumstances.
You have a Dual CS5000 in your other system. Have you tried swapping the tables? While I'm not much of a fan of the CS5000 either, it certainly is designed for better quality sound reproduction than a Technics 1200. Perhaps if you tried those phono cartridges on the Dual that Larry is offering , you'd get a better grasp of what they offer.
Don't worry about the Denon DL-103 damaging your vinyl. The arm on the 1200 is a much bigger hazard than a Moving Coil cartridge will ever be. The DL103 is a good sounding cartridge...and the AT is indeed "zingy."
Sorry to make my first post a downer, but I am all too familiar with both of those tables, having spent the 80's slaving in upper-mid-fi shops. Nothing says "mid-fi" louder than Dual.
You have chosen the other elements of your hifi so carefully and deliberately, it seems out of place that your analog is so...(I'm trying to find the right word here), mundane. (I'm not sure I found it).
Good luck on your quest. Keep in mind that the thrill is in the chase.