Cimpliance Vs Cartridge loading

Typically, when you go to a lower numeric value for loading (going from 500 ohms to 100 ohms) you are increasing the loading and this does cut the high frequencies.  This tends to make the bass seem MORE prominent, but you are reporting a loss in bass slam, so this is somewhat anomalous.  I think you are hearing increase in bass as bass being a bit loose.  

Your "free" option, at this point, is to try adjusting the vertical tracking angle (VTA) of the cartridge.  This also affects tonal balance--higher angle (toward more of a tail up, nose down attitude) would mean more highs and a leaner tighter bass, lower angle would increase bass and lower treble.  There might be some new combination of loading and VTA that gets you closer to what you want.

Good luck on solving your issues.

What you could be experiencing is a cartridge / tonearm mismatch, not a loading issue.  IME, the Zu/Denon likes a high(er) mass tonearm to get that tight articulated bass.

Yes, a heavier arm (add a US nickel - 5 grams) to the headshell will help, along with some experimentation to the VTA! I own several variants of the 103 and they can be finicky to set up!

For the second time, compliance (of the cartridge) has nothing to do with loading the cartridge.  Every cartridge has an internal resistance or DCR.  The DCR is equal to the resistance you would measure with a meter, if you put one probe on the "hot" pin of the cartridge and the other probe on the "ground" pin of the cartridge, same channel.  (But you shouldn't do this, because the current provided by the meter could fry the coils in the MC cartridge.  Just rely on the manufacturer's number.)  For any MC, the DCR is going to be between around 1 ohm and 40 ohms.  This number affects what will be the load resistance that the cartridge "wants" to see when you hook it up to the phono stage.  For MCs, most phono stages offer values ranging from 100 ohms on up.  In order for the audio signal to travel into the phono section from the cartridge with max efficiency at all audio frequencies, the input impedance (largely determined by that load resistance) ought to be about 10X higher than the internal resistance or DCR of the cartridge.  When you go below this rule of thumb 10X ratio, you begin to lose some of the audio signal to ground; this preferentially affects high frequencies at first.  Thus, when the load R is too low compared to the DCR of the cartridge, you hear that the sound gets "duller".  When the ratio between the two impedances is greater than 10X, e.g., a cartridge with DCR of 10 ohms driving a phono stage with an input load resistance of 100 ohms or higher, you should be ok with regard to flat frequency response within the audio range of 20 to 20KHz.  That's really all you need to know.

When you use a SUT with an MC, that's a different story.

I have a ZU/Denon 103 on my technics SL1200g TT and have it set at 200 ohms.  I have great bass slam and a nice sweet treble. I am using a Mac C2500 preamp which offers on the fly cartridge loading.