You make a very good point, there, Maarty! I got a new phone earlier this year and had to check just now. Nope! I don't have a 1/8" output!! That must be why I also got some Bluetooth earbuds about that same time. Yeah, that's the ticket. I sure did.
I must confess that I don't stream audio to my RS40 very often. Although I do have a 1/8" to 1/8" cable handy (apparently from my old phone), I usually just play songs out loud on the PC or TV in my music room.
B. Whoa! I looked up that BT speaker and found a Marshall W
oburn II and III and holy moly!! As amazing as I find the RS40 as a bass amp, I certainly would not expect it's secondary BT streaming functionality to
replace such a high dollar dedicated BT speaker!!!
ii. Too bad your phone (probably) can't stream audio to the Marshall while also running the Tone App for the Fender Studio! But, instead of replacement, maybe a tablet, or an old phone, could stream to the Marshall and you'd get the best of both systems?!?
(I have a couple of old phones that work great as mini-tablets via my home wi-fi.)
3. As mentioned above, I don't use the BT audio streaming at all and rarely use the cabled Aux In on either my RS40 or my R500c, either. But, when I do, I'm not too fussy about the audio quality, but rather more about my bass tone. So, my opinion on this is questionable.
f. However, I have read quite a bit here about
what a difference the horn makes, if present, for playing audio thru a Rumble. IIRC, most opinions are that the
horn being on helps the audio, but detracts from the bass tone. It begs the question whether to turn it on or off when doing both.

Aha! That answers my original question. I wish you the best of luck in today's practice.
And, again, "I feel ... your pain" in dealing with this techno inconvenience. I find such shortcomings in every single e-device I try/use.
[sigh] ... maybe someday ...
And, did this crash happen while following
7ender's
demand recommendation? If not, perhaps it was again due to the App/streaming conflict, instead?

No worries. I, for one, see such in-depth discussions as the
raison d'êtrie of such forums! Such detailed reports help
every interested party learn to improve their experience, maybe even
7ender, themselves! Who knows, the
Next Generation Rumble Studio may do as you wish!
Furthermore I, for one, would love to hear more about your adventures with the
7ender Mustang Micro! Being a 6-string guitarist myself, I considered mentioning it in my letter to Santa Claus. Now, for the life of me, I can't remember why I didn't!!!

Nobody/nothing is perfect. Life is compromise. Unfortunately, part of "getting to know" anyone/anything is learning to deal with their/its perceived shortcomings. An overall net positive experience is a win! You mentioned a husband, so you already know this.

Therefore, congratulations ... on both counts!
And please do keep us apprised here of your findings and progress.
.