I've got the SansAmp RBI and a BBE BMax T. I like having them both. For marching band, jazz band, and the other bands where i double on BG and DB, it's nice to be able to leave one preamp setup for upright work and one setup for electric. Really reduces switch time to me. [i'm 18 and a senior in high school]
There is a huge [to my ears] tonal difference between the BMax T and the RBI. I haven't [still haven't even after all this time] sat down and really just fiddled with the tonal settings on the BMaxT. I've found a sound that works for me, i modify it a little bit here and there-but i don't think i know how to currently/can't get the same degree of tonal variety out of it that i can the RBI.
With that being said, if it wasn't for the price that i got the BMax T for-i'm not sure if i would have ended up with this preamp despite the praise it's received on here-not a negative thing, but just since i don't know if i would have paid $600+s&h or tax for this VS. an Ampeg SVP or Billy Shehan, etc for however much they go for.
I bought my Bmax T off of someone on here for $300 shipped and i bought my SansAmp RBI off of a guy here for i forget how much-less than $250 i believe.
If you just play bass guitar [no doubling]-i don't really think there'd be that much of a reason beyond you wanting another pre, to get one. I'd probably leave the space open and then fill it with either the tuner or the glorified power strip err Conditioner. I do want to buy a conditioner and then get some really short AC cables so that i can clean up the inside of my rack a bit-rather than having a power strip with various lengths of AC cord hanging out the back of my rack.
As far as the tuners go-i've always just used the little Korg CA30s or whatever they're called. The hand held ones. I think a rack mount tuner would be nice, but for me-it doesn't make so much sense.
Compressor-i can't comment on as i don't use a compressor. The one on the BMax T, i leave disengaged.
A vent panel or other type of filler, if you want it-go for it, i'd probably just soon leave it empty than put one of those in.
That's all