Dual 721 turntable - opinions/advice?

I've has a Dual 721 for about 15 years. When I took is out of the plinth to replace the RCA cables, I couldn't help being amazed at how clean it is. This turntable had been sheltered all of it's life...a life of more than 40 years!
I'm not having any trouble with it but recently decided to upgrade the entire sound system. I started with a SCHIIT phono preamp. Schiit Vadar amp and Magnapan speakers should be here this summer but in the meantime, I bought a Fluance RT 85. It comes equipped with a Ortifon Blue cart. I was very impressed with the cart but the turntable had some sort of grounding issue on one of the channels, so I sent it back. I then decided that, being so impressed with the M2 Blue, that I would upgrade to the Ortofon Bronze and install it (with a new cartridge sled) on my Dual 721.
Here is where I am seeking advice and counsel.
Either my ears decieve me or the M2 Bronze is being muted in some way on the Dual 721. I'm wondering if, at a certain point, it is possible to hit the wall on a Dual 721 with cartridge upgrades!
If true, I wonder what aspect of the Dual 721 is driving this limitation?
Assume the tonearm to be balanced and the vertical tracking angle good, what other tweaks could translate to getting maximum benefit from this cart?
I did notice that the output of the M2Blue is 5.5 mV and the M2 Bronze is 5.0 mV, but the preamp certainly makes up the difference.
Does anyone have any thoughts?

Define “muted”.

I'll try.
Keeping in mind that at the point that I heard the 2M Blue on the Fluance RT 85, I had been used to hearing the the Dual 721 with a Vessel A3SE mounted on it. I only had the 2M Blue for a couple days but the color words chosen by Ortofon seem appropriate. The Blue was bright and precise....at first test, the acustic guitar on Beggars Banquet came out from behind the wall and even at low volume (especially at low volume) seemed to be in the room. At the next listening session I tried Strauss' Also Sprach Zarathrustra and Rimsky-Korsakov's Scharazade. The treble was sharp and the base precise. I heard individual stikes of drums instead of boom, even with the limitations of speakers that need to be replaced. When I compare specs of the Vessel with those of the 2M Blue, there is no doubt that the brilliant and precise sound I was hearing from the 2M Blue was objectively a better reproduction of sound..... The difference between the view of a landscape on a foggy morning versus a view of the same landscape under a crisp azure sky. 
After ordering a new cartridge sled and mounting the 2M Bronze on my Dual 721, I expected to appreciate another leap ahead in response. The difference was not that obvious. Of course, I was not comparing oranges with oranges. The 2M Blue was mounted on a different turntable and the 2M Bronze on my Dual 721. Did my ear acclamate to the 2M Blue after only two listening sessions? Was it more that I was expecting to objectively notice some aspect of the performance to be superior? The 2M Bronze, relative to the blue, sounds capable but not brilliant....that could be the difference of 0.5 mV in output? 
I have no doubt that the 2M Bronze was the right decision but I do hold to my original question. I wonder if the Dual 721 will supress the perceived performance of cartridges beyond a certain point and, if so, what is that limitation caused by.
I obsessed for a couple days over the stylus rake angle but after ascertaining to the best of my ability that the tonearm is parallel to the record surface and after getting photos of dynamic tracking, the rake angle alas looks fine. I have the Ortofon test record on order. 
I think the Dual is currently adjusted appropriately but if there is any problem with the azimuth, hopefully it will be identified by use of the test record.