Tim-iii
!!! I have no doubt whatsoever that the Rumble 25
could do it for you. At the same time, I have no doubt that the Rumble 40
could quite possibly do it even more satisfactorily for you.
Both could benefit from some
internal acoustic lining material to improve the cab's tone, since neither come with any.
A. The 10" speaker over the 8"
may be a significant improvement in tone to some users ears. The combination of bigger speaker and more power often tends toward better tone, in general. Search your feelings on this, balanced with the $100 price difference, if you can't actually try them both out.
2. The
controls are some the biggest differences in the Rumbles 15, 25 & 40. The 40 has the full control set of the bigger Rumbles. The 25 has many fewer controls. The 15 has fewest.
iii. The
40 weighs 3 lb.s less than the 25, despite being a bit bigger overall
! It's true. And, I wouldn't think the
slightly larger box would make any significant difference in the perceptions* of your Bluegrass friends.

The difference may
not be as big as it could be from this comparison. But, then again, maybe it could be. You never can tell 'til you try.
*I love playing with a large Bluegrass/Country jam group. They have no problems with my electric bass guitar and Rumble Studio 40. One of the multiple-time state fiddle champs there has invited me to play more with him if I would get an EUB, 'cause the contests don't allow
any amplifiers (period). I just dunno if I wanna make that commitment at this stage of my life.
.