Wow, where to start... I've begun writing a reply several times only to realize that I'm basicly regurgitating what has already been expressed here by those with many more years of experience with much more gear. It hadn't really occurred to me that I have an excellent opportunity to A/B these awesome amps through matching cabs with the ease of push/pulling two master vol knobs and swapping an instrument cable. The only thing I'm lacking is free time, which is at a premium these days with a teenager and 3 more under 10 to keep me busy.
Just to lay down my basic thoughts on the matter is to say that I am so fortunate to have started out with the 100T if for no other reason than that I would never have been compelled to get one after playing through the SB300. This is in no way meant as a slight to the 100T, it's just that the SB300 brings the weight and meat to every note that just absolutely does "it" for me. The difference is something I feel more than hear, if that makes any sense.
When comparing the amps in my mind, I can't help but use non-quantitative adjectives that are less than academic. I find that tonally, they are extremely similar, but the SB300 has so much more presence(?) which is more than just an increase in decibels. I believe I may now understand the concept of headroom a little more clearly.
Besides the fact that both of these heads are well on their way to becoming iconic bass amps, I will never, ever part with my 100T. It is not a little brother to the SB300 but an extremely versatile tone machine with a bag of tricks unique unto itself. Where the SB300 has ultimately provided me with my desired "voice" as a bassist, the 100T and a Squier Bass VI have expanded my creativity 10 fold in ways that border on addiction.
With time permitting, I would love to offer the opportunity for me to attempt to provide a comprehensive comparison at specific settings to those inclined to request.
Totally understand and have been there with the time and children situation etc ....
I'll agree with everything (that I've experience with )you state above...
100% agree re the presence and weight behind every note with the SB300 . In my experience , it's unparalleled . As I've stated more times that I can remember, I was all set to grab myself an SVT-VR, as it was THE amp that had impressed me so much... like no other had . TB'er, author, and respected bass gear hoarder

Tom Bowlus suggested that I try the (then new) Fender Super Bassman .... as I had seen the amps he'd used, liked most etc... I was taking his suggestions onboard

Now, I'd recommend the Super Bassman just as highly, of not more highly, than the VR. It's tone is at LEAST as good. . and then you've got another channel ... with a blendable overdrive and sweepable mids. Plus it's lighter in weight. And, for me at least, here in Aus at the time, it was a grand cheaper . (I think it's closer in price now as the Aus $$ falls back down).
But, yep, agree again. Being able to play notes on my "skinny li'l" G string

and know that they still punch hard

is to me, SUCH a bonus. And I've heard no other amp do it as well. Allows me to play more elaborate lines (without going silly of course.... I AM still a "meat and potatoes" bassist

) and still keep that solid cushion under the band.
I've tried plenty of amps since I first started playing in bands in the late 70s... and no amps do it as well. None. Not my mates DB750, not any 2kW power amps I've tried, no high powered class D amps. . Nuffink!

So... yeah. . I'm still as glad I got it as the first time I got it.