Well I am happy to hear it is working out :-)
One correction the Stock Audio Mirror amps utilize much better than Solen caps stock, they have Sonicap Gen II caps.. Which are also excellent, very neutral, very smooth sounding caps..
However the Jantzens are in fact a step up, at a minimum 10 - 15% better across the board in all aspects of superlative audiophile terms pick and choose... soundstage, low freq., Air, liquid, warmth etc....And they just do everything right as rain.
The Russian tubes supplied are a little drier compared to the NOS tubes you can get for the Audio mirrors, but still excellent as well, so for the standard Audio mirror package are still a great rounded out design and deal over most others.
The Isoclean fuses do make the amps a bit more sparkley and faster.. However power cords and pick your flavor have some good effect as well.
And by the way reason your plate amps will react to the change is these tubes have a much lower damping factor and better frequency range over the esoteric congested, and constipated Solid state stuff you might have been using.
Fact is just like adding a Good Tube preamp thats more or less exactly like what your plate amp is seeing with the signal transfering from the amps now, they are feeding it a wider and bigger sounding less Damped bass sound just as if you had a preamp with a bass knob and cranked it up, but its not because obviously these are doing it purely on the original tone of the design and that they go much deeper and bigger sounding anyway... Again almost the same effect many find with tube preamps and why the like them with combination of solid state amps in front of them.
Also funny thing is these little S.E.T amps driving Full on Passive subwoofers or woofers are the best punchy, clean, No ONE note bass Monsters I have heard, they do it with shocking authority, we tried them out on 12" subs direct and they easily sounded more tonal fuller and scarier than some 500 watt digital amps! So believe it or not they do even better driving woofers direct sometimes than feeding a powered system in the bottom end.
I am not sure how deep the full range is tuned in the Zu presence but you could disconnect your sub plate amp and just drive the Zu passive FRD and tweeter and I don't doubt you will get a very full sound with bass notes that match the plate amp, however not with all that tuned and isolated output of the subs.