Yes, I see what you did there. You guys are clever. That’s why I hang around here.

I’m glad to hear that you’re enjoying your Duck Dunn. It sure is a looker.
I’m all for discussion of how to get the most out of a Fender Japan bass.

I do appreciate the help.
Thanks again. You really have me thinking. A Bandmaster into a 410 Neo. Hmmm. And some sort of an A/B switch out of the Bandmaster to flip between the guitar and bass cabinets, depending on which instrument I want to play at any given time.
I definitely like the idea of getting good gear up front, avoiding future upgrade cycles. It would be great if my Bandmaster could serve as a good head for a while, given that I am not (yet?) gigging. So this 410 Neo should be that good for a cabinet, eh? There are a zillion options out there to choose from and so few that you can actually try in person. Nothing by Rumbles and Peavey combos at my local shop. The GC an hour away doesn’t have much, either. Not sure what the Sam Ash an hour away has on hand. And so I may end up buying something unheard, based upon recommendations like yours. Maybe get a good cabinet now. Worry about a more powerful head later (like the 100t)- if it’s ever needed for gigging.
It sounds as though a 40 Watt Bandmaster into the 410 would have good enough punch for home and small gigs, such that I could get a sense of the fun bottom end, eh? Hmmm. Any reason to think about going in any other direction for building up a bass rig? Bandmaster coupled with any other cabinets?
I would suggest doing some internet research into finding out if any other players have tried this pairing of the 40 watt Bandmaster with the current Fender 410 Neo Cab. You are correct in the assumption that if the 25 watt mode on the 100t can drive the 410 that your 40 watt Bandmaster easily could do the same.
As for an A/B switch to jump back and forth between amps... Just as long as the impedance if both head's speaker output is the same ohmage, then that should be fine. Just don't do it with the amp running one or the other.

As long as the amp is totally off (not even in Standby) I see no problem with matched impedance between your existing cab and the 410. But the thing is...You might end up preferring the 410 for your guitar rig as well! As I mentioned a few replies back, I have been running my guitars through my 100t/410 rig more and more lately.
While I get a great vintage sound with my VOX AC15 twin or AC30, the 100t is actually a killer guitar amp in its own right! (It bears a very striking similarity to a Marshall 100 watt tube head after all!)
In terms of auditioning before you buy? Wouldn't that be awesome! Living about 10 miles south of Yosemite NP, I have to drive 2 hours to the nearest store that sells guitar strings! lol! Finding a store that would actually have a high end rig like mine on the floor would be a miracle. I do nearly 100% of my guitar and studio gear purchases online based only on written reviews and the occasional You Tube demo. And of course the opinions of people I rust here on TB!
Speaking of which... I bought my 100t/410 Neo rig sight unseen and unheard too! I bought the 410 cab over the other available Neo configs based on all the rave reviews that the 4x10 model was getting in the 100t owners thread here:
Fender Bassman 100T Chapter 3 | TalkBass.com
Not surprising the 4x10 has proven to be the most popular of the line. You find that same popularity with other brands that offer various cab configs. A 4x10 enclosure hits a lot of check boxes for a wide range of players and studio owners. I personally consider a 4x10 speaker enclosure to be the ideal choice for the Fender Jazz Bass in particular. And since Jazz Bass remains my instrument of choice most of the time, I went with a rig that could give me the best Jazz Bass reproduction possible. It delivered that in spades! Far surpassing my expectations, which were VERY high to begin with! This rig purchase remains one of the most satisfying gear investments I have ever made in my life!
While I am sure others in this thread and elsewhere might provide you with dozens of alternative choices to the 410 and ultimately the 100t, If you start heading down that road, you are likely going to end up confused and trapped in a feedback loop of choices. I kind of got the impression based on our various conversations that you were more of a "cut to the Chase" type guy like I am! If someone else has already done all the leg work for me, and I trust their opinions and/or see the value of their professional experience, I tend to follow their lead rather than adventuring out on my own and risk making a bad choice. Which as we all know costs money!
And my precise intent in this case was to simply recommend you buy the same rig that I own. Either both at the same time, or starting with the 410 first with the ultimate goal of bringing a Bassman 100t in eventually. By then your 410 would be nicely broken in, and you would hear that awesome Jazz Bass sound that only the 100t/410 seems able to reproduce! And you would hear that on your first day of owning a 100t!
I confidently cast my vote for the same rig that I have owned and continue to LOVE for three years now. I am so pleased with. And as I said... My high praise for both of these components is based on my 40+ years of professional playing and recording. Having owned a commercial recording studio from 1979 through 1991, I went through a ton of gear, and also had the opportunity to play through and record with hundreds of amp and speaker combos that came through my facility with my wide range of clients.
The studio owned several different Fender Bassman models as well as Marshall 50 watt and 100 heads, paired with your typical Marshall cabs. Also owned several Ampeg rigs including a vintage SVT for Bass and a V4 for guitar. Some of the other brands I have owned and/or had on semi-permanent loan for studio time included several of the more popular Mesa Boogie head/cab systems as well as combos. The original ORANGE 100 watt tube head paired with a Marshall 4x10 or 4x12 cab, and the list goes on and on.
I doubt there are many TB members here with more ear time on so many different amp/speaker systems than I have worked with in my life. They would have to be recording engineers or studio owners like myself in addition to being a bassist, or be a member of a touring group that also regularly releases an album every year.