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I have the same problems with my CC-horn. I guess the tendency to collect condensed moisture is common with all brands of instrument, so you shouldn´t blame it on the Miras in particular.
It´s a matter of the wrap the manufacturers choose for their horns, though:
My F-tuba has self-emptying valves: No valve tubing bows below the valve level, so any moisture from the tubes will be drained into the main tuning slide whenever a valve is operated.
That´s a great system, but doesn´t seem to be applicable on the long-o-phones pitched in CC or BBb. Plus, I frequently need to empty the bottom bows of the CC-tuba, leading to french-horn style tuba-juggling that truly gets the audience´s attention .
Additional water keys would be more discrete, but then all the fun will be lost.
Seriously, manufacturers have reasons to struggle against additional keys. They will officially state that water keys will harm the horn´s intonation, but I guess it rather is a matter of cost.
At least I´ll know it´s time to quit playing tuba and start collecting stamps instead, when I´ll eventually drop the horn.

my king 2341 has a tendency of collecting a lot of moisture.

i think the most important thing is emptying all the slides before the horn is put away at night.

nothing ruins a morning of practice like the gurgling sound of water in the main tuning slide

The fourth valve is looooooong, so it will hold more warm, moist air, so it condenses more water. I've noticed that the valves on my tuba collect water almost directly proportionally to the length of the valve tubing. All I have is a 3-valve student model, so I don't have a fourth valve, but I have pretty much the same situation with my third. The fourth valve might collect less, since it's used less often, but since it's used on low notes with slow, warm air, I can imagine it might make up for lost time.
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Joe Baker, who doesn't use his third valve nearly as often as his second, but dumps three times as much water from it anyway.

I bought an old horn off E-Bay and brought it to Schmidt's Music Store in Pensacola.Dave Schmidt recommended adding push button water keys to the first and third valve slides and boy am I glad he did.I try to swallow each time before I blow but I still accumulate lots of liquid in the slides.The two extra water keys help a lot.I'm tempted to add one in the tiny second valve slide but if I blow real hard I can move that water to the third valve slide.Also holding the second valve down while I give it the "King Spin"empties the second valve slide.

My VMI stencil collects all the water in the 4th valve too. My theory: gravity. The 4th valve is on the bottom on the stack, so the water simply gravitates to it. i came to this conclusion because my french horn has the same "feature". All the water collects in the 3rd slide on the Bb side, which is the lowest of the lot, and it even collects there if I play an entire session on the F side!

Yes, my 186 has the spit problem in the 4th valve, too. Every 2-3 numbers, I need to dump it; every 5-6 I need to do the spin-o-rama to get the spit out of the other (lower) bow in the 4th. That does attract a bit of attention, as someone else mentioned!
Awhile back, Joe detailed exactly where to have a water key installed, but I foolishly did not bookmark the post and now can't find it. Anyone smart enough to save it?