Monitor Cab OFF effecting Speaker Out signal? - and - What is best live set up to maximize KPA quality via Speaker Out?

I'm sure this has come up before but I can't seem to find a thread for it so could really use some help. Please forgive my general ignorance as I'm sure much of this post will reveal I'm a bit daft when it comes to stuff that other guitarists have known for decades. I could really just benefit greatly from some confirmation or explanation on some of this...

I have a powered profiler and have been playing it through a Marshall 4x12 for live settings. Not the ideal scenario, I know, but it's still been good. A couple sound guys balked at running the KPA to the house PA and just preferred to mic up a cabinet "the old fashioned way".

I've been trying to figure out exactly how to improve my live sound and what gear to acquire to do so and did a bit of research here to get things going.

The 2 things I was considering were

A) Getting a stereo power amp to drive 2 4x12s in a stack or L/R split configuration in order to get the stereo quality sound from the delay and other features of the KPA (to be honest though, myself and the sales guys remain perplexed why the Powered KPA is rated at 600w/300w when it only has one speaker output and can't drive 2 cabinets without running through an additional stereo power amp).

B) Getting one passive or two powered FRFR monitors in order to remove the color of the Marshall from the equation and get a more accurate replication of the intended sound of all these great profiles.

So I go to the local music shop and rent a Yorkville NX35 passive monitor, bring it home, disconnect the 4x12 from the Speaker Output and connect the Yorkville and it sounds kind of dead and flat. Noticeably worse than the Marshall ever did. I'm surprised as I was expecting the opposite but figure it might be something in Output settings. Everything looks fine then I toggle MONITOR CAB OFF from dim/unchecked to dark/checkbox checked and it seems to brighten up a lot.

Again I'm surprised as I thought this was only intended to effect the signal from the Monitor Output. So quite confused on that as the manual states:

"The MONITOR OUTPUT allows to run the amp sound without virtual cabinet, while other outputs still contain the virtual cabinet. Engage “Monitor Cab Off” to switch off the cabinet. Any Output Source can be applied to the MONITOR OUTPUT with “Monitor Cab Off”."

***EDIT*** - So, I've since realized that the Monitor signal is what's being sent to the Speaker Out so, while it now DOES make sense that toggling MONITOR CAB OFF would effect the signal, it seems to be doing so in the opposite way that I would have expected. Shouldn't Monitor Cab Off technically make it sound worse instead of better? Feeling really dumb on this one I must say : \

I'm sure that I'm missing something major on both these matters and look forward to someone setting me straight so thanks a lot in advance. Much appreciate all replies!

TLDR: In short, aside from running the Main Outputs to the house PA, what's the ideal scenario for getting great stereo replication of profiles via the powered KPA? And, why would the Monitor Cab Off feature effect the quality of tone from the Speaker Output?

Not sure if no responses due to 'figure it out yourself noob', poorly stated question or no one's sure but I think I got it figured out so will reply myself and if I'm wrong maybe someone will correct me.

For one I realized my question was poorly stated and based on bad intel because I hadn't clued into the fact that to simply turn off the cabinet element of a profile, the logical first method to employ is on the front of the unit itself in the stack section, not via the output settings.

I also realized that the monitor and speaker out signal being identical (with the exception of one being powered and one not of course) meant that 'Monitor Cab Off' also meant the same thing as 'Speaker Output Cab Off'.

Once these clicked, I realized that one might logically wish to have the cabinet profile active for the main outs but not for the monitor so this would be the time when one would leave the cab of the profile active on the front panel but turn it off via Output Settings.

Yes? No? I think so

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As for the first part of my post, I rented a passive FRFR stage monitor and tried that out and it sounded great but my lack of experience showed when I went for some sweet vibrato feedback for the intro of King of the Nighttime World at our show last night and did NOT get good results. Lesson learned. Stage monitors have tweeter horns and are not going to respond well to having a guitar brought too closely to them. As this is something I need to do at our shows, I'll pass on the FRFR option until I can find a pair of affordable FRFR 2x12s or 4x12s made for guitar. They don't seem to be readily available here (yet?) so we'll see.

What I did do was rent an ART SLA 2 for a month and am running 2 4x12s off of that and it sounds great so that will be my physical rig moving forward for now.